Chalcedony

A crypto-crystalline sub-species of quartz. It is generally of a milky brown colour, but there are also rarer blue (blue chalcedony), green (chrysoprase) and orange-red (cornelian) varieties. Very occasionally it may be transparent.

Agate is frequently stained to resemble types of chalcedony; so particular care should be taken when buying chalcedony that you are not, in fact, about to be sold an imitation.

The mineral is found in Madagascar, Brazil, India, China and the USA.