Chalcedony

Formation

Chalcedony is a form of silicon dioxide, composed of delicate intergrowths of quartz and morganite. It is a dense, semi-transparent mineral occurring in a large range of colour varieties. Pure, uncontaminated chalcedony appears homogeneous and is white, grey or blue. When illuminated from the back, it may display red hues. Chalcedony can be found in weathering volcanic rocks, but also in sedimentary ones, often together with agate. In magmatic or metamorphic rocks chalcedony is rather rare and only forms veins in fissures that have been percolated by warm, silica-rich brines. Sporadically, chalcedony can be found in fossils, as a petrifying agent.

Prehistoric Origins

Chalcedony was in use in the Mediterranean region since at least the Bronze Age (3,000 BC – 1,200 BC); archaeologists in Crete have discovered set of chalcedony seals dating back to circa 1,800 BC. In the east, a great many of the carved gem intaglios and ring bezels have been found along ancient trade routes in Central Asia. Several fine examples of objects made from the stone in the first century A.D. were also recently unearthed in north-western Afghanistan.

Seals and Victorian Jewellery

Due to its unique chemical structure, hot wax does not stick to chalcedony, therefore in the old times the stone was widely used to make signets and seal rings that allowed to imprint bearer’s seal or coat of arms in wax on important documents and correspondence. Historical chalcedony seals can be found anywhere in the world, from the Middle East, through Germany and France, to the United States. The mineral and its varieties were widely used in the make of Victorian jewellery, with particular preference towards moss agate, bloodstone, and onyx.

Biblical Lore

The Bible mentions number of chalcedony varieties set in the foundation of the city of Jerusalem (Revelation 21:19-21), and in the High Priest Aaron’s breastplate (Exodus 28:15-30). According to the Jewish tradition, Aaron’s breastplate was supposed to represent the twelve tribes of Israel.

Sources

Chalcedony is mined worldwide and German town of Idar-Oberstein has been its largest processing centre since the middle of the nineteenth century.

Variations

The most noteworthy types of chalcedony are:

  • Agate
    Characterized by either transparency or elaborate colour patterns. Opaque varieties of agate are sometimes referred to as jasper.

  • Moss Agate
    Variety of chalcedony characterised by green filament-like inclusions, giving it the superficial moss-like appearance.

  • Chrome Chalcedony
    Coloured by chromium compounds. It is also known as ‘Chiquitanita’ when found in Bolivia, and ‘Mtorolite’ when found in Zimbabwe.

  • Chrysoprase
    Chrysoprase (also spelled chrysophrase) is a green variety of chalcedony, coloured by nickel oxide.

  • Carnelian
    Clear-to-translucent reddish-brown variety of chalcedony. Its hue may vary from a pale orange, to an intense almost-black coloration.

  • Heliotrope (Bloodstone)
    Green variety of chalcedony, containing red inclusions of iron oxide that resemble drops of blood, giving heliotrope its alternative name of bloodstone.

  • Onyx
    Variant of agate characterised by black and white banding. Correspondingly, agate with red, brown, orange and white banding is known as sardonyx.