Learn · Gemstones and pearls
Hue, tone and saturation
Gemmologists describe a coloured stone's colour with three numbers and letters: which colour it is (hue), how light or dark (tone), and how strong or greyish (saturation). Build a colour and read it the way a grading report writes it, for example slpR 6/5.
Choose a hue, then move tone and saturation to see the grading code.
Read the guide
Birthstones around the world
Every month on the American, British, Japanese and nine-gem lists
Where gems come from
A world map of where each gem is mined, today and historically
The 7 pearl value factors
GIA's seven pearl value factors, changed one at a time on one pearl
Gem treatments explained
Usual treatments gem by gem, the trade codes, care and disclosure rules
Cutting angles and light
Change a stone's cutting angles and follow the light: where it returns and where it leaks Preview