3 Computer Drive West, #122 Albany, NY 12205
alphagemlab@aol.com          Fax 518-489-2776
By appointment:  Mon-Tues  9:30-4:30.

                           Closed Wednesdays, Fridays and weekends.

Alpha Gem Lab

Colored Gemstones

Colored gemstones fall into three categories:

  • Natural gemstones - those mined out of the ground
  • Synthetic gemstones - those made in a laboratory having the same chemical properties as natural stones from the ground
  • Simulant gemstones - those made to simulate another gemstone

As with a diamond, colored gemstones are graded using the 4 C's, although color is the most important C here because it is the first thing that you notice about the gem.

COLOR:  We describe color as the hue, tone and saturation of the stone.  Hue is the actual color that you see.  Is it red?  Does it have a modifying second color so you would call it purplish-red or orange-red?   This would be written as "R", pR, oR.

Then we look at the tone of the stone.  How light or dark is it on a scale of 0 to 11?   0-1-2 are so light that it looks white and 8-9-10-11 are so dark it just looks black. The ideal tone is in the 4-5-6 range.

Saturation is described using a scale of 1-6.  How pure is the color?   1-2-3 include some gray or brown, making for a dull appearance.  5-6 on the saturation scale are bright, vivid colors. 

In grading colored gemstones, we also look at the brilliance-window-extinction factor, adding up to 100%.  Brilliance is the bright sparkle that meets your eye.  This should be the largest percentage of the three.  The window is usually in the center of the stone.  Does the light enter and go straight through (like a window) allowing you to see a pen or finger behind it?  Extinction refers to dead dark areas.

SHAPE:  This is the outline of the gemstone. It can be emerald-shape, pear, oval, round, square, heart, marquise, fantasy, oval cabochon, or round cabochon.  Grading the shape takes into account things like odd measurements that require special mountings, sides not parallel,  sides not evenly matched or misshapen, corners that are too narrow or too wide, undefined points, and flat or bulged sides.

CLARITY:  Gemstones are classified according to type:

Type 1 (often inclusion-free):                               Type 2 (usually included):                                                                  Type 3 (almost always included): 

Beryl -Aquamarine, Morganite, yellow                       Andalusite                                                                                                       Beryl - Emerald

Chrysoberyl - green, yellow                                            Chrysoberyl - Alexandrite                                                                           Tourmaline - red, pink and watermelon

Quartz- smoky                                                                   Corundum (sapphire and ruby), all colors

Spodumene - Kunzite                                                      Diopside

Topaz                                                                                   Garnets - all colors

Tourmaline - green                                                           Iolite

Zircon - blue                                                                       Peridot

Zoisite - Tanzanite                                                            Quartz - citrine, amethyst, amethyst-citrine


                                                                                               Tourmaline - blue, green, orange, yellow, bi-colored                                                                                                  

                                                                                                Zircon - all but blue

CARAT WEIGHT:  Actual scale-weight or determination by recognized formula for the type and shape of the specific gemstone.

Gemstones can be enhanced to stabilize or change their original color.  They can be bleached, oiled, heated, irradiated, dyed or impregnated with a substance to make them more durable.  Some treatments are widely recognized as "normal" treatments  while others are performed to fool the potential buyer.  These can have an effect on the value of the stone.  Not all treatments are considered permanent and may fade or dry out with time.  Not all treatments can be detected with a glance or even with use of a microscope. All should be disclosed by the seller.  When in doubt, the gemstone should be sent for testing to an international laboratory such as GIA. 

Another service that the international labs may provide is "country-of-origin."   In some cases this will also impact the value of a stone.


January -  Garnet (all colors)                                  July - Ruby

February - Amethyst                                               August - Peridot

March - Aquamarine                                              September - Sapphire (all colors)

April - Diamond (all colors)                                  October - Opal

May - Emerald                                                          November - Topaz

June - Alexandrite or Pearl                                   December - Zircon (Tanzanite on some lists)