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


I'm sure you've heard of the four C's of diamond grading.  Let me remind you what they are:

CUT:  This refers to two things. The first is the shape of the diamond: round-brilliant, pear-shape, marquise-shape, oval, princess, radiant, emerald, Asscher, heart, cushion, Old European cut, Old Mine cut, triangle-brilliant, baguette, or single cut (18 facets).   Most have an arrangement of 58 facets.  For instance, a round-brilliant has 1 table, 8 bezel or kite facets, 8 star facets and 16 upper girdle facets on the top (crown) of the diamond.  It has 16 lower girdle facets, 8 pavilion main facets, and 1 culet on the bottom (pavilion) of the diamond.  The girdle separating them can be faceted, frosted or polished.

The second part of cut analysis refers to proportions and how they relate to the theoretical "ideal."   Components are:

  • total depth percentage
  • table percentage
  • crown angle
  • pavilion depth percentage
  • girdle thickness
  • culet size
  • polish and symmetry

Fancy shapes have additional criteria: the length-to-width ratio...

  • rectangular shape = 1.50 - 1.75 to 1
  • marquise shape = 1.75 - 2.25 to 1
  • oval shape = 1.33 - 1.66 to 1
  • pear shape = 1.50 - 1.75 to 1

Heart and triangle = 1.00 to 1

CLARITY:  The size, number, shapes, color and location of inclusions (flaws) in the diamond when viewed through ten-power magnification. 

The Gemological Institute of America (GIA) developed this grading scale:

F - IF = flawless and internally flawless;   VVS1 - VVS2 = very very slightly included;   VS1 - VS2 = very slightly included;    SI1 - SI2 = slightly included;    I1 - I2 - I3 = included.

Inclusions should not be visible to the naked eye until the grade reaches I1, and possibly SI2 (for a person with exceptional vision).

COLOR:  GIA also developed a color grading system using the letters D through Z.   Following grade Z,  diamonds are considered Light Fancy, Fancy and Intense Fancy.  The D-Z grades are divided as follows: D-E-F = colorless;  G-H-I-J = near colorless;  K-L-M = faint color;  N-O-P-Q-R = very light color; S-T-U-V-W-X-Y-Z = light color.  Diamonds come in red, orange, yellow, green, blue, purple, brown, black and grey colors.   The color can be natural - or enhanced.

CARAT WEIGHT:  Carat weight can be determined by placing a loose diamond on a scale, or by use of a recognized mathematical formula for the specific shape of the stone (used to estimate weight of mounted stones).  Diamonds are typically priced according to their carat weight.  There are 100 points in a carat just as there are 100 cents in a dollar.  One-quarter carat = 1/4 ct =.25 ct = 25 points.   One-half carat = 1/2 ct = .50ct = 50 points.  Three-quarter carat = 3/4ct = .75 ct = 75 points. 

Enhancements:  Diamonds can be enhanced to improve their color or their clarity.

Enhancements include:

-  fracture filling with a substance that is pulled into the diamond through use of a vacuum.  This is not considered a permanent enhancement and should be disclosed when the diamond is sold.  Sale price is typically 40-60% less than that of  a non-enhanced diamond.

-  laser drilling to reach and excise a dark inclusion.  This drill hole will typically be smaller than the width of a human hair and is considered a permanent enhancement.  It should also be disclosed by the seller.  Typical selling price is 20-25% less than that of a non-enhanced diamond.

-  Irradiation to change color to a fancy color

-  HP/HT (high pressure-high temperature) which subjects the diamond to conditions similar to those it was originally subject to when it was first formed deep underground.  This is a permanent enhancement that can lighten the diamond's color.

There are also many synthetics sold in the marketplace that look like diamonds.  These include CZ, Moissanite, white sapphire, stronium titanate and white spinel.  

Diamonds can be sold with a laboratory report from an international lab such as GIA, European Gemological Laboratory (EGL-USA, EGL International), International Gemological Institute (IGI), HRD, GCAL (NYC) or American Gemological Society (AGS).  These reports typically provide a quality analysis of the diamond and/or an origin-of-color report to determine natural vs enhanced color.  Remember: GIA developed the grading system for both color and clarity.  The other labs use the same terms, but may interpret them differently.