Agate

Agate is a variety of chalcedony, itself a microcrystalline variety of quartz. Chalcedony is generally translucent, but there are also frequent cases in which it is opaque with a waxy-vitreous lustre, it is semi-hard.

Its colour is very variable, from white to blue, red, green, brown and black.

One of the characteristics of some varieties of chalcedony is a change of colour when exposed to gradual increases in temperature. It forms in various types of rock, but more frequently within lava rocks. Its formation is favoured by relatively low temperatures and by silica-rich solutions.

More specifically, agate is characterised by curved or concentric bands of different colours, generally pale colours in shades of grey, yellow, brown and rarely green or blue.

Agate is often used to make spheres or rings and sometimes intaglios and cameos.