Weights and Measures:
1 Grain = .327 Carats or 65.36 miligrams (mg)
1 Carat = 0.2 Grams (200 mg) or 3.06 grains
1 Gram = 5 Carats or 15.432 grains
1 Ounce = 28.35 Grams or 141.75 Carats
1 mm = 0.1 cm or 0.0394 inches
1 cm = 10 mm or 0.394 inches
1 in = 2.54 cm = 25.4 mm
Eye Clean - When the gem is carefully inspected under a bright light and turned all directions there are no visible inclusions.
Clean (for cut gems) - When the eye clean procedure is done, but with 10X loupe and there are no visible inclusions.
Clean (for rough) - Same as Eye Clean unless the gem allows closer inspection through a clear "window".
Birefringence / Double Refraction - The ability of a material to split a single light ray passing through it into multiple rays on emerging from the material. A gem with this property will look like it has more facets than what is actually cut.
Dichroism/Trichroism - This is the ability of a mineral to show different colors when viewed at different angles. (Dichroic = 2 colors, Trichroic = 3 colors)
Dispersion - The ability of a material to break up white light into the individual spectral colors (like a prism).
Photochroism - This refers to the ability of a mineral to change color when viewed in different lighting conditions. (Alexandrite is the prime Color Change example)
Refractive Index (RI) - The ability of a material to bend the light passing into it. Air has an RI of 1, Diamond has an RI of 2.41; so a Diamond will bend light almost two and a half times better than air. RI is a very important measure for gems as it affects how the gem should be cut and how brilliant it will be when cut at the proper angles.