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Agate is a variety of Chalcedony (Quartz) that has a variegated colour. The natural colours being:
In intercalated bands distinctly marked from each other (called ‘banded agate’), the difference in the bands being due to degrees of transparency and colour and to inclusions of milky layers alternating with layers of Quartz, Jasper, Cornelian, Onyx, Sardonyx, or Sard; or: in cloud-like, moss-like, or dendritic form, due to coloured inclusions or dendrites as in moss agate(also called Mocha agate), cloud agate, feather agate, and mosquito agate.
Most banded agate is artificially coloured; the process, for black or brown and white stones, involves steeping in a solution of honey or sugar, then soaking in sulphuric acid and applying heat so that the carbon is released and enters the absorbent parts, colouring them blackish. This is an acceptable practice if the colour is fast and the trade description correct.
Agate was used extensively in Egypt and Rome, in ornamental pieces and in jewelry such as beads and brooches; in the Middle Ages it was worn as an amulet or talisman.