YAG
YAG is an actonym for yttrium aluminum garnet, a man-made imitation diamond. This imitation stone lacks the fire of a natural diamond.

Yellow Gold
Yellow gold is gold that has been alloyed with a mix of 50% copper and 50% silver.

Yowah Nut
A form of Boulder Opal from Yowah (Queensland), Australia. It occurs most often as nut-sized ironstone nodule containing pockets and veins of vivid precious opal.

Yttrium
A silvery metallic element of the boron-aluminum group, found in gadolinite and other rare minerals, and extracted as a dark gray powder. Not a rare earth but occurring in nearly all rare-earth minerals, used in various metallurgical applications, notably to increase the strength of magnesium and aluminum alloys.