Diamond Glossary

The knowledge and more information you have the better diamond purchase you will make. Here are a number of diamond terms that are used by jewelers to describe diamonds and which appear on diamond grading reports.

T:

Table:
The flat facet on the top of the diamond. It is the largest facet on a cut diamond.

Table percentage:
The value which represents how the diameter of the table facet compares to the diameter of the entire diamond. So, a diamond with a 60% table has a table which is 60% as wide as the diamond's outline. For a round diamond, gemologists calculate table percentage by dividing the diameter of the table, which is measured in millimeters (this millimeter measurement does not appear on diamond grading reports) by the average girdle diameter (an average of the first two millimeter measurements on the top left-hand side of a diamond grading report). For a fancy shape diamond, table percentage is calculated by dividing the width of the table, at the widest part of the diamond, by the millimeter width of the entire stone (this total width measurement is the second of the three millimeter values in the top left-hand corner of the diamond grading report. Contrary to popular misconception, having a small table percentage (53% to 57%) does not make a round diamond any more brilliant than a diamond with a larger table.

Trilliant Cut:
A type of brilliant fancy shape that is triangular.

Tone:
A color's position on a colorless-to-black scale.

Treated Diamond:
A diamond with a bodycolor induced by some form of artificial irradiation, often in conjunction with controlled heating (known as annealing).

Twinning Wisp:
A cloudy area produced by crystal structure distortion, usually associated with twinning planes.

Table Size:
The size of the table of a fashioned diamond, expressed as a percentage of the stone’s narrow-girdle diameter, is a dimension used in proportion analysis. On a round brilliant, it is measured from corner to opposite corner, rather than from flat side to flat side.

Top Cape:
An early trade term still used by some dealers to designate the diamond color grade between crystal and cape in the river-to-light-yellow system. Small stones in this range will face up colorless when mounted, but larger stones will have a yellow tint.

Trigon:
A triangular indentation occurring as a growth mark on diamond octahedron faces. The sides of the trigon are reversed with respect to the face on which it occurs.

Twinning Lines:
Visible line on or with in a fashioned diamond, caused by twinning in the crystal. Since the orientation on one side of a twin plane differs from that on the other, the best polishing direction for one is a poorer one for the other; as a result, a line remains at the surface. Also called knot lines.

Toughness:
The ability of a mineral or gemstone to resist breakage (fracturing). Toughness is not the same as the hardness of a stone.

Transparency:
The ability of a substance to transmit light.

Treated Gems:
A general term used to described gemstones that have been artificially modified to improve their color or clarity. Techniques include laser drilling, fracture filling (clarity enhancement), high pressure high temperature (HPHT) annealing, irradiation, and surface coloration. Gemological laboratories have the equipment and experience to detect the more difficult treatments.

Trillion:
A triangular shaped and faceted gemstone. More specifically, it is a gemstone that has an arched shaped triangle cut with 25 facets on the crown and 19 facets on the pavilion.

Twin Crystals:
Two or more mineral crystals that have grown together in a symmetrical nature, usually nonparallel. Sometimes called twinned crystals.

Troy Ounce:
A weight unit for precious metals predominately used in England and the U.S. Twenty pennyweights (dwt) equals one troy ounce, and twelve troy ounces equals one troy pound.

Thermal Conductivity:
The ability of a material to conduct heat. Diamonds have the highest conductivity of any material.

Top