Precision-cut gemstones perform far better than not-so-precision-cut stones. 

When you buy a gemstone, it is the best to see the stone in person.  But for an eBay shopping, you need to rely on pictures provided by sellers.  This guide explains how to judge the cutting quality by looking at those pictures.  If the seller is not showing enough pictures, ask them questions before buying. 

I am a faceter.  Everyday I work on to bring out the beauty and brilliance of a given stone.  It is surprisingly simple to tell precision-cut stones from not-so-precision-cut stones. 

Looking at those faceted gemstones on eBay, I believe that the cutting quality is the most neglected aspect of gemstones. On the other hand, the most over-rated aspect is the carat weight. I'll touch on this later.  

I want more people to know the cutting quality makes a big difference and precision-cut gemstones are far more brilliant and incredibly vibrant.  That is why I am writing this guide. 

Gemstone Terminology:

Here are some of the names of the gemstone parts. I need to use these names in the following guide.  This is a side view of the classic round brilliant cut.  These names apply any cut design of a faceted stone. 

• The widest part (shown as 2) is the girdle. 
• The part below the girdle (shown as P) is the pavilion.
• The part above the girdle (shown as C) is the crown. 
• The top flat part (shown as T) is the table.  There are tableless designs also.
• Each flat surface is a facet.  These include the girdle.
• Each point where four or more lines meet (for example, 2, A and B or 1, 2 and 4) is a meet point.

What to Look for:

The Reflection Pattern of a Precision-Cut Stone is Very Neat.  

Just look at overall reflection patterns from the crown.  Precision-cut stones look very neat and geometric, even through the table.  If not, there are two reasons.  The stone has many inclusions.  Or the pavilion is cut at wrong angles, showing a window or fish eye.

Precision-cut stones

There is no geometric pattern because the stone has many inclusions or it is cut in wrong angles.

Conical Pavilion vs. Bowl-Like Pavilion 

The key to the brilliance of a gemstone is in the pavilion.  Those sparkles come from light entering from the crown and reflecting back from the pavilion facets.  To do this, a cutter must take a certain angle into consideration, when he cuts each gem material. It is called the critical angle.  A conical pavilion cut at angles slightly above the critical angle gives brilliance most effectively.  On the other hand, a bowl-like pavilion is a product disregarding this optical property.  Stones with a bowl-like pavilion gain more carat weight sacrificing the beauty and brilliance.  Many commercial stones from abroad are cut this way.  American faceters call this type of cut "belly cut". 

• Since gemstones are priced by the carat weight, heavier stones sell at higher prices. That is one reason why many stones are belly-cut. 

A cone-shaped pavilion (re-cut of the right stone) and a pavilion that looks like an inverted bowl

• If you have a chance to take a look at a stone in person, here is the easy test to see if the stone is precision-cut.  Just look straight down on the TABLE placing the tip of the stone on prints.  You should NOT see the prints below.  If you can read them, the light is passing through the stone, not reflecting back.  The four pictures below and above are the same stone.   

A stone with no window (re-cut of the right stone) and a windowed stone

If the center of a stone appears dark without showing a geometric reflection pattern, the stone is cut too deep.  The light is leaking from the side of the stone.  It is called a fish eye.

About the critical angle

The critical angle is an optical property of a stone.  When the light ray hits the flat surface of a stone at lower angles than the critical angle, the surface reflects the light back.  On the other hand when the light hits the surface at higher angles than the critical angle, the light passes through.  This is the reason a stone needs to be cut at certain angles to get the most reflection, i.e., brilliance.  

The critical angle varies depending on the types of stones.  The critical angles of common gem materials are usually between 35 degrees and 45 degrees.  For example, one of the higher ends is Opal (43.6 degrees) and the extreme of the lower ends is diamond (24.5 degrees).  You don't need to learn these numbers, unless you facet a stone yourself.  But remember this: a stone with a pavilion that is too shallow or too deep/steep shows a window or fish eye.  A bowl-like pavilion is too steep near the rim and too shallow in the center.  

About a very flat crown

I see quite a bit of gemstones with a very flat crown on eBay.  If the pavilions are cut correctly, they still perform OK.  But if the pavilions are too shallow or belly-cut, they show hideous windows.  The height of a crown (C in the above gem drawing) should be somewhere between 1/3 and 1/5 of the height of the pavilion (P in the above drawing).  An exception is a type of cut with a very high crown, which is called a dome-cut.  Look at a stone from its side.  Choose a well-proportioned stone like the drawing above.  The tip of the pavilion should be roughly 90 degrees or a little bit more.  If the tip of the pavilion is less than 90 degrees, it is too pointy and there is a greater risk chipping the tip.  

Faceted Girdle vs. Ground Girdle  

The girdle is a very important part of a gemstone.  Facets are placed from the girdle tier by tier just like decorating a tiered cake.  If the girdle is not precisely cut, it is very difficult for a cutter to cut the rest of the stone producing tight meet points.  A precision-cut stone must have a faceted girdle.  A rounded ground girdle is a sign of mass-production.  Those facets and meet points of mass-produced stones are usually very much uneven for this reason.

A rounded ground girdle (left) and a faceted girdle (right)

A top view of the round brilliant cut.  Examples of tight meet points and even facets (left) and sloppy meet points and uneven facets (right)

In summary, pay attention to:

1. How is the overall reflection pattern of the stone?  Is it neat and geometric? 

2. Can you see the neat reflection through the table as well?

3. What is the shape of the pavilion?  Is it conical?

4. How is the overall proportion of the stone?  Is the tip of the pavilion roughly 90 degrees? 

5. Is the girdle faceted?  How are the facets and meet points? 

If the answers are all yes, you found a superbly-cut gemstone.  Congratulations!  This stone must be sparkling to its fullest potential.  If 4 yes out of the 5 questions, it is still a well-cut stone.

I encourage you to take a look at gemstones posted on eBay for practice.  You will quickly become good at telling well-cut stones from "yikes" stones.  You will also find it is not necessarily the pricier the better-cut.  

If you learned something new and useful from this guide, please click "yes" below.  Thanks!!