However, the term prase is also used to describe green quartz and to a certain extent is a color-descriptor, rather than a rigorously defined mineral variety.)

[11] It can display a wide range of iridescent colors including red, orange, yellow, green, blue, and purple.

A light blue variety of chalcedony is known from Illorsuit, formed in the volcanic rocks along the southern coast of the island.

[13] Chalcedony was used in tool making as early as 32,000 BP in Central Australia where archaeological studies at sites in the Cleland Hills uncovered flakes from stone brought in from quarries many kilometres away.

[15] Chalcedony was used for green and yellow color in prehistoric cave paintings, for example at the Bhimbetka rock shelters.

The chalcedony was ground to powder form then mixed with water and animal fat or tree resin or gum.

Fine examples of first century objects made from chalcedony, possibly Kushan, were found in recent years at Tillya-tepe in north-western Afghanistan.

The term chalcedony is derived from the name of the ancient Greek town Chalkedon in Asia Minor, in modern English usually spelled Chalcedon, today the Kadıköy district of Istanbul.

(Jewish tradition states that Moses' brother Aaron wore the Breastplate, with inscribed gems representing the twelve tribes of Israel.)

Originally the agate carving industry around Idar and Oberstein was driven by local deposits that were mined in the 15th century.

In addition, cheap labor and a superior knowledge of chemistry allowed them to dye the agates in any color with processes that were kept secret.

Possible reasons include the existence of the moganite component, defects caused by Brazil twinning, and small crystal size.