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Named for the Achates river in Sicily, agate is a variety of microcrystalline quartz. Most specimens exhibit alternating bands of light and dark colors. When the bands are wide enough, solid color stones can also be present. There are dozens of agate varieties on the market. The following is a list of the better known types in ancient and medieval times:

  • chalcedony (uniform in color—in the narrow medieval sense, refers only to pale blue and gray varieties)
  • cornelian & sard (also known as carnelian—a variety of chalcedony ranging from pale orange to scarlet, when heat-treated the stone turns blood red; sard is the reddish brown variety)
  • chrysoprase (an apple green chalcedony; see also the entry on emeralds)
  • dendrite or "moss agate" (milky white mottled with olive green)
  • heliotrope or "blood stone" (teal green with blood red spots; when these spots are absent, the stone is referred to as plasma)
  • onyx (alternate bands of creamy white, beige, and pale brown or tan)
  • sardonyx (alternating white and ruddy brown; while sardonyx does occur naturally, most specimens on the market have been dyed in a process known as caramelisation—the onyx is boiled in honey or a sugar solution. The tan layers take on a rich brown color, while the denser white layers are unaffected)
  • black onyx (also known as "touchstone", with alternate bands of white and jet black or gray; nearly all black onyx is dyed—following the sugar dye process, the stone is carbonised in sulphuric acid, turning the brown layers black)

See Also