A precious ornamental stone that is a form of chalcedony, a variety of colored quartz.

Most agates form as nodules in stratified deposits of silica found in certain rock cavities. The agate layers vary from clear to opaque, and they assume many shades of color because of the presence of microscopic particles of iron salts. The colors appear in combinations of yellow, brown, gray, blue, or black, and these may be attractively distributed in patterns of stripes, bands, or cloudy blends. Agate is slightly harder than steel and can be polished to a high gloss.

Agate used by the Israelites in the wilderness may have been brought from Egypt. According to Pliny the Elder, red agates veined with white were found in the vicinity of Thebes. Such a red agate may have been the variety that was mounted on the high priest’s “breastpiece of judgment” to represent one of the 12 tribes of Israel. The center stone of the third row on Aaron’s breastpiece was an agate (Heb., shevohʹ, a kind of precious stone).​—Ex 28:2, 15, 19 (ftn), 21; 39:12.