Chalcedony
A microcrystalline silica used in tools, gems, and art since prehistory.
Chalcedony is an aggregate of cryptocrystalline or microcrystalline silica, composed primarily of quartz with minor amounts of intergrown moganite. It has a waxy luster, is semitransparent or translucent, and occurs in a wide range of colors, most commonly white to gray, grayish-blue, or brown. Chalcedony has been used since prehistoric times for tools, pigments, and ornamental objects, and many semi-precious gemstones—such as agate, carnelian, chrysoprase, and onyx—are varieties of chalcedony.
- composition
- SiO2 (quartz and moganite)
- crystal_system
- Trigonal (quartz) and monoclinic (moganite)
- luster
- Waxy
- transparency
- Semitransparent to translucent
- common_colors
- White to gray, grayish-blue, brown
- notable_varieties
- Agate, carnelian, chrysoprase, heliotrope, onyx, sardonyx
Lore & Background
The name chalcedony comes from the Latin chalcedonius, probably derived from the town of Chalcedon in Asia Minor, though the Online Etymology Dictionary notes a connection with the town is 'very doubtful'. The name appears in Pliny the Elder's Naturalis Historia as a term for a translucent kind of jaspis. Another reference to a gem called khalkedón appears in the Book of Revelation (21:19), but it is a hapax legomenon, making it uncertain whether it refers to the same mineral known today.
Reader's Guide
It was ground into powder for prehistoric cave paintings, as seen at the Bhimbetka rock shelters. In the Bronze Age, chalcedony seals were used on Minoan Crete, and later along Central Asian trade routes it was carved into intaglios and beads showing Greco-Roman influence. Its property of not sticking to hot wax made it ideal for seal impressions. The mineral's many varieties—including agate, carnelian, chrysoprase, heliotrope, and onyx—have been prized as semi-precious gemstones. According to tradition, at least three varieties of chalcedony were used in the Jewish High Priest's Breastplate. The term 'plasma' is sometimes used for green translucent chalcedony. Commercially, chalcedony is often enhanced by dyeing or heating.
Did You Know?
- Chalcedony is composed primarily of quartz with minor amounts of moganite, a quartz polymorph that crystallizes in the monoclinic system.
- The name chalcedony may derive from the town of Chalcedon in Asia Minor, but the Online Etymology Dictionary calls that connection 'very doubtful'.
- Heliotrope, a green variety of chalcedony with red inclusions, is also known as bloodstone.
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