Rough Evaluation

Buy as a skeptic, sell as an optimist!” – J.B.

There are certain desirable traits in Faceting Rough. Mainly, one wants Good Color, Good Shape, Good Size and Good Clarity.

Good Color:

“Pretty” is in the eye of the beholder, but generally, the eye will automatically find the most expensive stones. That demand, across many viewers, is what makes a given facet rough expensive – demand. Color is 60% or more of the value in any colored stone, so look at color carefully, in various kinds of light. Stones will likely appear differently under fluorescent than incandescent or sunlight…

Good Shape:

Generally, you want a faceting rough stone that’s “blocky” or “chunky” or “rounded”. Flat or twisted shapes or faceting rough with deep pits or valleys will yield very poorly. Here’s an example of a badly-shaped facet rough:

 

The long, pointed end on the left of this will waste, as will the thinly flattened end on the right. Only a very small portion of this stone can be recovered by faceting.

Good Size:

Everyone has a claim about the yield of a gem rough. Facet rough dealers often say you’ll get a 1-carat stone from a 2-carat faceting rough. And, some cutters claim such feats on a regular basis, but any cutter who keeps records of actual weights knows the truth – that the industry standard for commercial cutting is a 20% yield (an 80% loss of weight) is average.

Pencil everything conservatively, using the 20% yield, and you won’t go too wrong. If you want a 1-carat stone, then buy a facet rough of at least 5 carats (one gram) weight. If the dealer is selling by-the-carat, then divide by five to find the likely finished carat weight. If they’re selling by-the-gram, then number of grams in equals number of carats out. (There are five carats in one gram.) This is fundamental in understanding what to expect from your gem faceting rough.

Clarity:

You want a stone that’s “clean” of inclusions and flaws – Flaws are cracks or voids or open cleavages or other things that compromise not only the appearance, but also the structural integrity. A flaw breaching the surface of a finished stone is doubly bad because it hampers light at the surface as well as internally. Inclusions are foreign matter within the rough gem. Such things absorb light and interfere with the optics of a gem.

A perfectly clean stone is difficult to find, and is not necessarily the target. The object is overall presentation and beauty – even if an inclusion is highly visible or even made into part of the presentation. However, in general, flaws and inclusions are things we like to avoid in the purchase, or to remove in the cutting.

Your Toolkit:

The “basic” toolkit can grow out of hand (and out of budget) quickly, so this setup is the very most basic, and can be assembled for under $35:

• Loupe ($10 and up, depending)
• Lamp with solid opaque shade ($10 and up)
• Fluorescent Light (probably free in your kitchen)
• Incandescent Light (probably free in your den)
• Sunlight (free)
• Tweezers ($5 and up)
• Glass of water (free, in your kitchen)
• White paper (free, in your kitchen or bathroom)
• Time, attention, and patience

All of the following critical tests and evaluations can be performed with the above, and some practice.

Your Gem Testing Checklist:

• Confirm Identification
• Check Size
• Check Shape
• Test Color
• Test for Flaws and Inclusions

Each of these items is important, and requires detailed explanation:

Confirm Identification:

It’s important to know what kind of stone you’re buying or selling. Identification of gem materials is a complex science, and a lifelong study. Know that your dealer knows the stones he’s selling you and learn all you can to protect yourself.

Here’s a photo of a crystal that a dealer sent as “Fluorite”:

Note the shape of the crystal is hexagonal.
(Fluorite does not make hexagonal crystals.)

Note the metal from a dinner-fork tine that I used to attack the surface of this stone. Note that the stone ground away metal – The metal did not scratch the stone. What do you think this crystal is?

Identification is an ongoing issue if you’re buying rough. Read, look, test, learn, repeat.

Check Size:

Size can be “bad” if the rough won’t support a stone that is marketable or that is “a good deal” after you count the cutting costs. For instance, a plain red Almandine Garnet rough weighing one gram will finish a gem weighing about one carat. Retail price for a one-carat commercially-cut red Almandine is far less than I’d charge to cut it. So, unremarkable red Almandine Garnet in one-gram sizes is not an investment. It seems size really does count…>

Check Shape:

Roughs that are broken or shaped with long, skinny points, flat areas, or divots should be avoided. They will yield very poorly.

What color is oval?

Color can be directional (read more below). Make certain to notice how direction of color may affect the usefulness of a rough’s shape! For more information on shape issues, study my page on geometry.

Test Color:

Color can be “bad” in three different ways:

• Too Dark
• Bad Dichroism, or a Closed C-axis
• Bad Color Modifiers

These things require a detailed explanation, so let’s take each one in order:

1. Color Too Dark:

You’ll find most rough dealers on the Internet photograph their pieces with strong back-lighting. Back-lighting shows clarity, but is deceptive with respect to color. Stones that have nice color when back-lit will often be too dark when faceted. Detect the “too dark” problem using the “white paper test“.

This stone may look beautiful here, but click on the photo to learn about the test that will prove it’s a real dud – and why you should not buy stones based on the color you see in such photographs.

2. Bad Dichroism, or a Closed C-axis:

Many stones are dichroic, which means they show different colors in different directions. Some gems with this ability will show nearly identical colors in all directions, while some will show only differences in color density (darker or lighter). However, some will show drastically different colors – such as red in one direction and green in another. This can be a prized feature or it can be a nightmare.

This stone may look beautiful here, but click on the photo to learn about the test that will prove it’s really yucky – and how to guard against buying worthless stones like it.

Which way is red?

Imagine that a stone has a strong red color (desirable) in one direction and a weak pink color (not as desirable) in another. This is typical of Oregon Sunstone! Now, imagine that the stone is much better shaped for yield in the direction that shows weak color, and very skinny in the direction that shows the desirable strong red color. Again, this is typical of Oregon Sunstone! In stones that have directional color (Sunstone, Iolite, Sapphire, Tourmaline, and many more), an interplay of shape and color-direction will drastically affect yield and value! Research the specific gem you’re interested in, and look carefully at how color and shape relate before you spend!

Tourmaline is one stone that often has extreme dichroism, a variation of which manifests as “closed C-axis”. This means that light will not pass through one axis (direction) of the stone at all. Because light must pass in every direction for most gem designs to sparkle, the closed C-axis condition can render a Tourmaline worthless, and in almost any case should drastically reduce the price of the rough stone. Detect the “bad dichroism” or “closed C-axis” problems by using the “Look all around for color test“.

This stone may look beautiful here, but click on the photo to see how simply looking from a different angle will prove you shouldn’t buy it – or any others like it.

3. Bad Color Modifiers:

Colored gems show the main color you see when you first look, but when you look closely you may notice the secondary color or tint – a modifier. This modifier can be any other color from grey to yellow, brown, etc. If the modifier is an unpleasant color – or if the combination of the modifier and the main color is unpleasant, then the price of the stone will drop accordingly. Remember that color is 60% or more of a colored stone’s value. Detect the “bad color modifier” problem by using the “white paper test” and the “Look all around for color test”.

Test for Flaws and Inclusions:

These problems are more common in some stones than others, but should always be searched for. Flaws can cause a stone to come apart during cutting – resulting in a broken heart as well as a broken stone! Flaws can seriously impair the presentation, interfering with light transmission or causing unwanted reflections, etc., and drastically affecting clarity, which is a huge factor in stone value.

Here, a flaw deep within this Beryl (arrow) is reflecting light brightly. Can you spot any of the other flaws in this stone?

Inclusions can be found in almost any natural stone – if you use enough magnification. They can be a problem when eye-visible to the casual observer, or when loupe-visible to the connoisseur – depending on your reason for buying the stone, and the level at which you want to play this game. Not all inclusions subtract from value. Oregon Sunstone often benefits from decorative schiller inclusions, and Demantoid Garnet is more desirable if it has “horse-tail” inclusions.

This Tourmaline crystal shows many inclusions. Click the image to learn how to do this test properly.

The above exceptions aside, we’d prefer stones that don’t have eye-visible inclusions because they usually detract from presentation and value. However, some kinds of stones such as Emerald, for example, nearly always have some inclusions. Do some research and know what to expect in the specific gem you’re shopping for! Detect inclusion problems using the following three tests:

• Side-Lighting
• Candling
• Water

Gem Testing Techniques:

How to Check Size:

Look up prices, and check reasonable weights for the size stone you want. (Colored stones are measured in millimeter sizes, but sold by the carat.) Use a reasonable reference source for your research. If you’re shopping garage sales, then use a garage-sale-grade market such as e-bay. If you’re shopping quality custom gems, then shop custom cutters. If you’re shopping retail-commercial-grade stones, then visit your local jeweler.Find out what a stone the size and cut you want will probably weigh. Multiply by five to find the weight of rough you’ll need to shop for. Don’t shop for a pint if you really need a gallon!

Look All Around (for color, NOT inclusions):

Focus on one thing at a time! If you’re doing a color survey then keep color on your mind and forget all else for the moment. Look through the stone from every direction – especially if the stone you’re shopping is known for directional color. Look in every direction, using different lighting sources.

If you’re testing for color direction, you can use back-lighting, but remember to use the “white-paper test” for color quality and density. Take special note of any directionality to the color – and examine how that relates to stone shape (remember yield).

This Tourmaline looks like it could make a beautiful Pear (left photo) – until you check the other direction (right photo) – and see the dark, bile-green color. If you cut this stone for size alone, it will face-up this ugly color. If you cut the stone for the smaller pear-shape, the beautiful teal will be polluted with enough vile overtone to drastically affect the appearance and value. This is what we call leaverite…

Even if the proportions were reversed, and the pretty color were in the long dimension, there would still be concern about the presence of the darker, and less-pleasant color on the C axis. The following photo shows a finished gem with such a condition.

Even though the design of this gem was created to maximize the desirable color, one can clearly see the darkening and yellowing toward each end of the stone. Whenever shopping Tourmaline or other strongly dichroic gems, always check the desirability of color in every direction. Reject stones with undesirable colors on the C axis as you would closed C axis (unless the stone is long and pencil shaped as in this study).

If you’re shopping a Tourmaline, insure that the C-axis is open – or that the shape will allow you to work with the stone even if it isn’t. Always look in every direction to insure you’ve checked well.

Two views of the same Tourmaline crystal – one showing beautiful color, and the other showing the closed C axis. This stone cannot be cut to produce a good-quality gem.

White-paper test (for color, NOT inclusions):

Stay focused on one characteristic of the gem at a time. This test is for color ONLY. Place the stone on a white sheet of paper and observe it at a 45 degree angle while lighting from a 45 degree angle from behind. Reflect light off the paper and through the stone toward yourself. Do this test with all three kinds of light (incandescent, fluorescent, and daylight). DO NOT lower the angle to pump more light through the stone – Remember to buy as a skeptic and sell as an optimist.

Here are two photos of the same rough – One in the aggressively back-lit style of most Internet dealers, and the other in “white-paper test” lighting that shows the amount of color you can realistically expect after faceting. This stone “fails” the white-paper test – no sale!

If light reflects off the paper and through the stone then this is the approximate color and color-depth that you’ll get from a stone in this piece of rough. If the stone is “dark” then walk away. It doesn’t matter how pretty the stone is with back-lighting, you’re never going to see that color in a faceted stone.

Examine the color and quality of the light coming through. Look carefully for undesirable modifiers, and turn the stone about to see if different directions may show different colors. Consider dark directions and unwanted colors carefully – buy as a pessimist!

Here are a pair of Aqua crystals. The one on the right shows reasonable color, while the one on the left is “sick” – both pale and green – neither of which are desirable in Aquamarine. Forgetting size, price for these crystals should be quite different.

A parcel of small red Garnet rough showing color and density. The brown modifier isn’t too bad, and the density is acceptable for commercial material. This parcel “passes” the white-paper test.

This Amethyst crystal has much waste on the long, tapered end, but it has very nice color in the thicker area. It’s so dark we were concerned, but the white-paper test shows that good color will show after faceting. This stone is a winner!

Although this Amethyst crystal is “pretty”, the strong zoning will present a graduated intensity of color when the crystal is cut into a single long gem. An evenly-colored crystal (and resulting gem) of this size and shape will be much more rare than this obviously zoned one – thus worth more.

CAUTION: Some Internet dealers show only white-paper photos, which is very good for determining color and density (60% or more of stone value), but poor for showing clarity. A photo from one direction also does not show color direction, which can be critical in gems like Tourmaline. When the only photo shown is a white-paper shot, you may wonder what’s going on with the clarity. Is the stone included?

CAUTION: Some Internet dealers show only back-lit photos, which is very helpful for examining clarity, but poor for showing color quality and saturation. When the only photo shown is a back-lit shot, you may wonder what’s going on with color density. Is the stone too dark?

CAUTION: Some Internet dealers place rough stones on white cardboard that has holes punched in it so they can back-light portions of the stones from beneath. Look carefully for rounded bright spots in photographs and you’ll discover the dealers who attempt to save time by combining into a single photo the two most useful lighting techniques for rough evaluation.

However:

1. Back-lighting is most useful for detecting inclusions and internal flaws. So, back-lighting a small portion of the stone discloses only a small portion of the stone’s clarity, and may not be representative of the entire piece.
2. The whitepaper test is best for discerning post-cutting color saturation. So, pumping light in from behind a white-paper photo, even into a small area, may tend to artificially reduce the apparent saturation, making darker-than-optimal stones appear top-grade – and making too-dark-to-facet stones appear passable.

In fairness, photography is difficult, and time-consuming. And, the different lighting methods require different set-ups and camera settings. So, dealers tend to learn and use one photographic technique to present their wares. It is not my intent to insinuate dishonesty on the part of any dealer(s), but to alert the potential buyer to the inherent strengths and weaknesses of each presentation so they will know what to look for; what to ask for; and what to question before buying.

Look All Around (for flaws and inclusions, NOT color):

Focus on one thing at a time. If you’re surveying for flaws and inclusions then forget color – that’s a different job. Look through the stone from every direction. Use different lighting methods, and do a complete survey with each method before moving on to the other.

Side Lighting:

Hold the stone at the very edge of a light source so you can shade along the edge of an opaque shade or the edge of a flashlight. You can use a fiber-optic to direct light along thin paths and in specific directions. Side-lighting means left, right, top, or bottom – as long as the light comes into the stone from the side, and not toward you. Side-lighting usually causes inclusions to reflect light, so look for unexpected light spots in the stone.

The side lighting (from the top) on this Amethyst crystal shows many internal flaws, two of which are marked with arrows. This stone is not suitable for cutting.

Side lighting (top again) on this Beryl shows a storm of tiny bubbles within. Although this crystal, and internal features are quite beautiful, it is not suitable for cutting.

Side lighting on this Aquamarine crystal shows flaws (yellow arrows), a cloud of bubbles (green arrow) and an icy frost-like layer, which is obviously opaque (blue arrow). Depending on the style of cutting and the orientation, this stone may be useful after the cloud and frost are sawn off.

Note the many streaks within this Aquamarine. Features deep within the stone (and beneath the light) are reflecting light, and so are bright. Features that are closer to the camera are casting shadows, and so are dark. This stone will produce a “commercial-grade” gem.

“Candling” or back-lighting (for inclusions, NOT color):

Use varying intensities of back-lighting. You can also “pinch” the stone between fingers to shade light from glaring around the sides. Back-lighting usually causes inclusions to cast shadows, so when you’re candling look for dark spots.

This very-included Tourmaline is a good example of how inclusions cast shadows when back-lit. This stone is obviously unsuitable for cutting.

This Amethyst specimen is not even transparent, and back-lighting shows this, and the internal growth patterns both very nicely. This is a “tumbler-grade” stone.

Water (for flaws and inclusions, NOT color):

Water can relieve much of the surface glare to let you see internal features – especially on shiny or “frosty” stones like Garnets or alluvial Topaz. Stick the stone in a glass of water and then use side lighting and back-lighting. Use your tweezers to turn the stone and remember to look in every direction.

Water cuts the glare so the many inclusions in this Garnet are easily seen. This is not suitable for cutting.

This beryl is in water, with back-lighting that shows off inclusions readily. One is marked – How many more can you find?

This double view of the same stone shows how important it is to use different techniques and to look from different angles. Combining both techniques, notice how many flaws and inclusions are visible. This is a nice specimen, but unsuitable for cutting.

The following two stones are alluvial Topaz, with a very frosty surface. At first glance, there appears to be no way to discern whether these stones merit a second look.

…but watering and side-lighting easily show the serious problems with this stone, two of which are marked. Can you count the others?

Likewise, the second stone has a very bad veil running through the middle. Neither of these stones is suitable for cutting a single large gem, but the second stone would readily produce a pair of gems after sawing away the veil.

You may have surmised that presenting goods on “white-paper” emphasizes the quality of color, while not showing clarity. Presenting goods under back-lit conditions emphasizes clarity, while overstating the brightness (understating color density). And, neither of these lighting methods accomplishes what side-lighting does.

Any reasonable gem rough evaluation demands one use all three techniques. Remember to use all three techniques whenever you examine a rough!

Determining what kinds of flaws and inclusions are acceptable is something for experienced experts. If you have doubts, send your stone for evaluation. Meantime, learning to see these features is an important step.

Studies:

Following are a number of simple “case studies” showing a variety of evaluations made using the simple techniques above.

Amethyst, with flaws, inclusions, color-zoning, and growth-lines shown using watering.

Other than some serious striped color-zoning this large Amethyst crystal looks acceptable with plain back-lighting.

But, using side-lighting and water, we can easily see a huge flaw trailing through the middle of the stone.

The same stone, from different angles shows the severe color-zoning, and some very dark tendril-like inclusions, both of which would seriously degrade this stone’s appearance.

Watering is also fun just to enjoy the fascinating growth patterns!

Garnets with good color, and one stone with inclusions, demonstrating white-paper testing and watering.

Here’s a parcel of red garnet, which I would reject from the beginning just based on size. There’s no way I can make money cutting these little stones, but a hobbyist could have many hours of enjoyment with such nice color. Notice how the white-paper test shows the color and color-density of the parcel. Larger stones of the same density would be too dark…

Now that we’ve checked color and density, let’s change lighting and look for inclusions. Here’s back-lighting, and it looks like a few black inclusions.

Now, the same stone back-lit and in water – notice how the inclusions show up!

Tourmaline, with flaw, shown using back-lighting and then white-paper lighting.

The classic back-lit photo shows the flaw in this stone – and what appears to be wonderful pink color. But, we know not to use back-lighting for color…

The white-paper test reveals how dark this stone actually is – and an undesirable root-beer overtone, so this stone is rejected on two counts.

Smoky Quartz, showing color-density, clarity, inclusions, flaws – and demonstrating the white-paper test and watering.

This is what is meant by “glassy”!

The second crystal looks foggy, but much of that is surface-glare.

We use side-lighting and water to see the clarity of the upper portion; the inclusions in the base; and the bad flaw below the tongs. We can saw-away the flawed area and use the top!

Tourmaline Crystal, showing clarity, color, and C-axis and demonstrating white-paper lighting and back-lighting.

Back-lighting to search for inclusions, this Tourmaline “pencil” shows serious flaws near the ends, but a nice clean area in the middle. (The length-wise streak is on the surface of the stone.)

The same crystal in white-paper test for color. Notice the lightest area along the bottom edge where light is reflecting off the crystal and coming back out. This stone will show a nice bluish-green color when cut.

The same crystal, looking end-on against very strong back-lighting shows that this stone has a closed C axis, which would be a problem in a short, blocky stone, but isn’t that troubling in a nice long pencil like this. We can still cut an acceptable gem from this rough.

We’ll need to subtract value for the closed C, and the wasted weight on both ends, but given the right price this crystal will be a winner.

Tourmaline showing flaws, inclusions, closed C axis and shape issues, and demonstrating white-paper lighting and back-lighting.

When we candle this stone with back-lighting, we can clearly see a flaw appearing as a dark shadow.

Though the color looks great, the same flaw, deep in the center of the stone, is clearly visible due to the angle of the lighting. This stone would be rejected due to the flaw, but we want to complete the study just for practice…

Rolling the stone to test color from every direction, we see that the C axis is closed. Even without the flaw this stone would be a dud.

I hope you’ve found this lengthy page useful!

Please leave comments or questions below – or forward questions or suggestions, and I will be happy to answer (and upgrade the page).

Special Thanks to my colleague Jackson, for providing many of the Namibian gems pictured above, and for the questions that prompted me to create this page.