“The first Roman who wore a Sardonyx [a variety of Chalcedony] according to Demostratus, was Africanus the Elder. From then on, this gemstone has been held in high esteem in Rome.”
Pliny the Elder (23-79 B.C.), Historia Naturalis
The name Chalcedony is derived from Caledonia, an antique port in the Roman province of Bithynia, situated in present-day Turkey. The term ‘Chalcedony’ always generates confusion, as it not only refers to a family of minerals, but also to a particular member of this family.
In the broadest sense, it refers to all varieties of cryptocrystalline Quartz, but also indicates a subgroup within this family that only includes gemstones ranging from blue to white-gray. The multicolored varieties are classified as ‘Agate.'
Chemical composition of Chalcedony
Chalcedony is a Cryptocrystalline Quartz, specifically a silicone dioxide, composed of Quartz and Moganite. Its Chemical composition is SiO2.
Blue Turk Chalcedony, thanks to its deep transparency, has been mined since antiquity and remains in high demand today. Other deposits are found in Brazil, the United States (California), India, Madagascar, Malawi, Namibia, Sri Lanka and Zimbabwe.
The Chalcedony: the mother stone and raw crystal
The history of Chalcedony
Chalcedony plays an important role in many cultures, particularly ancient ones, such as ancient Mesopotamia (modern-day Iraq), Egypt and Greece, leaving traces in Christianity, Islam and Buddhism; for instance, It is the preferred stone for the manufacture of Buddha statues.
In the Bible, Chalcedony is one of the twelve gemstones given to Moses on Mount Sinai, which in obeisance of divine order (Exodus 28: 15-30) were embedded in Aaron’s breastplate (grand priest and brother to Moses). These gems represent the twelve sons of Jacob and the respective tribes of Israel. Four of these twelve gemstones are members of the Chalcedony family: Agate, Diaspro, Onyx and Sard (Carnelian).
Another group of twelve gemstones placed in the foundations of the walls of the city of Jerusalem is cited in Revelations (21, 19-21), and once again, four Chalcedonies are present. These gemstones are directly associated with the Apostles (Revelations 21, 14) and according to the classification of the Bishop of Caesarea in the 10th century, Chalcedony represented the Apostle Andrew.
The Romans were also fascinated by Chalcedony, and it was the gemstone of choice for magic amulets, seals, cameos and signets, and also for embellishing rings, pins, bracelets, brooches and necklaces.
Chalcedony’s fame extends beyond antiquity and into the Medieval period and the Renaissance. It remains a popular stone among gemstone cutters, jewelers and precious stone collectors, and as a material used in the decorative arts.
Properties of Chalcedony
Generally, Chalcedony’s transparency ranges from opaque to translucent, its striations are white, with rough and irregular fracture; it does not have a cleavage plane and has a hardness of between 6.5 and 7 on Mohs scale.
The most important criterion for valuating Chalcedony is color, which ranges from blue to milky-gray. Stones with uniform color distribution and greater transparency are the most desired. Chalcedony is usually cut in cabochon and used for engravings, cameos and carvings. Valuing the cabochon is simple - finish, shape and proportion are important. Where engravings, cameos carvings are concerned, personal preference becomes more important, but in general the complexity of detail and distribution of color should be considered.
Varieties of Chalcedony
- Blue Chalcedony
- Chrysoprace - Chrysoprase, also called ‘Green Chalcedony’, is a variety of Chalcedony with fascinating and beautiful green tints.
- Carnelian - Carnelian is a translucent or opaque variety of Chalcedony with a red-brown color.
Care for Chalcedony
If exposed to intense light, Chalcedony may lose its intense color or return to its original color; on the other hand, ultrasound and steam cleaning are perfectly safe.