Articles by Jean on Gemology

The Four C's Of Colored Stones --a general overview of rating colored stones, including an addendum with additional details for those in the trade
Watch Out For Windows! --helpful tips on evaluating how well a gemstone is cut

THE FOUR C's
OF
COLORED STONES

You're probably familiar with the four "C's" of diamonds, but you may not have heard as much about the four "C's" of colored stones. Colored stones include all stones except diamond, even though many other gemstones can also be clear and transparent.

COLOR

You are first attracted to a gemstone by its color. Color can been classified by:

HUE (red,green, bluish-green, etc.)
TONE (lightness or darkness)
SATURATION (intensity)

More about color

CLARITY

The second thing you see involves clarity. Clarity refers to:

BLEMISHES (on the surface)
INCLUSIONS (internal characteristics or irregularities within the stone)

Each stone is unique and its inclusions can be used as "fingerprints" to identify it. Unlike the standard grading system in place for diamonds, colored stones do not yet have a universally accepted system.

More about clarity

CUT

Cut does not refer to the shape of a stone, but to how well it is cut or its:

PROPORTIONS and FINISH

Colored stones are faceted into many different shapes, including the round brilliant, oval, emerald step cut, heart, marquise and pear. Whatever your preference, the importance of a well-cut stone is obvious! You can spot a well-cut stone because it sparkles with life! You don't want to see through the bottom of the stone! This is called a "window" and most stones with windows do not have much brilliance.

Well cut	Too shallow	Too deep
	More about cut

CARAT WEIGHT

Colored gemstones are weighed in carats as are diamonds. A one carat diamond is approximately 6.5 mm in diameter. Colored stone weights will vary due to the differences in the angles used in cutting and their specific gravity. Prices will vary greatly as well, just as they do with diamonds.

More about carat weight

In looking at all these aspects,
you can begin to gain more awareness and
a greater appreciation of the beauty and wonder of the mineral kingdom!

ADDENDUM TO THE FOUR C's

COLOR

GIA has developed a system that describes colors very accurately, using 33 hues, tones from 1 to 6, and saturations from 0 to 10. In addition to the dominant bodycolor of the stone, there can be additional colors, due to pleochroism (tanzanite), color-change (alexandrite), color zoning (tourmaline) and for other reasons.

Return to Color

CLARITY

Because of the wide variety of colored stones, three clarity types have been proposed for use with the GIA system. Type I stones can generally be found to be almost inclusion-free (such as topaz). Type II stones are usually included (such as garnet), and Type III stones are almost always included (such as emerald). Each stone is compared with others of its type, and is classified using eight categories from VVS (very small inclusions) to Declasse (inclusions interfering with the light so much that it is not transparent).

Return to Clarity

CUT

The sparkle is due to the stone's brilliance from being cut at angles which allow the light entering the crown (top) of the stone to bounce around in the stone, and return out through the crown, rather than absorbing the light, or leaking it out through the pavilion (bottom) or sides. Cut includes both proportions and finish. Good proportions include a pleasing face-up outline, brilliance and profile. Finish includes a smooth polish and symmetrical facets.

PROPORTIONS (relationships among crown, girdle, pavilion):

a. From the top:

outline

length to width ratio

evenness

is there a window?

b. From the side:

girdle thickness

depth of stone (2:1 to 3:1)

bulges?

symmetrical profile?

FINISH:

a. Polish:

girdle -- faceted and polished?

lines, scratches, pits?

b. Symmetry:

balance and consistency -- regularity of facets

check each tier -- do "meet points" meet?

facet junctions -- are they aligned?

c. Facet Survey:

number and size of facets -- too many, too few?

Return to Cut

Even more about cut

CARAT WEIGHT

The price variations are due to several factors, including the above mentioned color, clarity, cut and carat weight. In general, the rarer the stone, the more valuable it is. Some stones are readily available in larger sizes, so a 6 carat stone may be twice the price of a 3 carat stone. Others, such as ruby or tanzanite, may be 10 or more times the price.

Return to Carat Weight

WATCH OUT FOR WINDOWS!

EVALUATING THE CUT OF A GEMSTONE

Many people ask for advice on how to choose a gemstone for jewelry. We often hear about the 4 C's of Diamonds or Colored Stones: Color, Clarity, Cut and Carat weight. This discussion will focus on cut (make), not referring to the shape of the stone, but to how well it is cut.

G.I.A., one of the primary educational and testing organizations in the world, separates cut into two parts: proportions and finish. Proportions refers to the relationships among the crown (top), girdle (middle) and pavilion (bottom). First, look down at the top of the stone to determine if the outline is pleasing. Check the length to width ratio, evenness of each side, etc. Do you see straight through the top of the stone to the surface underneath? If so, this "window" is stealing much of the brilliance and color that a better-cut stone would offer you. The window suggests that the pavilion angle is too shallow for the gem material. Each gem material has a specific refractive index (RI), indicating the angle at which the light bends as it enters the stone. If the pavilion is cut using too low an angle, you will see right through the bottom of the stone (not a pretty sight in most stones). You will get the greatest brilliance in a gemstone if the light goes in through the crown, bounces around in the stone, and then exits back through the crown. This has been called "optical excitement". Is the table (the flat top) about 1/2 to 2/3 of the width of the stone?

Next, look at the stone from the side. Is the girdle thick enough to be set in a standard setting without being chipped (not a knife-edge), but not too thick to prohibit it from being set easily? Is the depth of the stone, as well as its proportions (crown height, girdle thickness and pavilion depth) pleasing, or is it too deep or too shallow? Generally the pavilion depth should be 2 to 3 times the height of the crown. This means that the ratio of the pavilion to the crown should be 2:1 to 3:1. Do either the crown or pavilion bulge out excessively? Is the profile symmetrical?

After checking the proportions, look at the finish. Finish consists of polish, symmetry, and suitability of facets. A good Polish includes (1) a faceted and polished girdle (many stones cut overseas have rough, rounded girdles), and (2) no lines, scratches or pits which detract from the overall appearance of the stone. Symmetry refers to the balance and consistency of the design and the regularity of the facets. Are the facets in each tier the same size and shape? Do they point up, that is, do they meet at the points where they should? Are the facet junctions sharp, or are they rounded and misshapen? Are the facets in alignment, that is, do they meet accurately at the girdle from the crown and from the pavilion? Facet survey checks the appropriateness of the number and size of the facets for that stone. Are there too many tiny facets in a small stone, or too few in a large stone?

If all this seems like too much to remember at one time, there are several things you can begin to do. First, look at a lot of stones at Gem Shows, jewelry stores and lapidary shops. Ask questions. Start with some of the basics:

(1) check for a window, (2) check the girdle to see if it's polished and faceted, and (3) look for sharp facet junctions. And be sure to ENJOY THE BEAUTY OF GEMSTONES, a gift to you from the mineral kingdom!

Top of Page

Home

About Us