Chalcedony

(kal-sed-n-ee)

 HISTORY and LORE

• Though not familiar by name, an old and illustrious gem family. Treasured by humans since prehistoric times, chalcedonies were sacred and royal gemstones in early civilizations around the world.
• Named for Chalcedon, an ancient trade center located in what is now Turkey.
• Long used in Europe as a charm against ghosts.

VALUE FACTORS

• Color: Range is almost unlimited. Many natural colors, and others from treatment.
• Clarity: Translucent to opaque.
• Cut: Cabochons, carvings, tablets, and beads in many shapes.
• Size/Weight: many varieties are available in all sizes, and occur in pieces big enough for large carvings or other ornamental uses. (Normally not priced or sold by carat weight.)

TREATMENT

• Often dyed, and in a wide variety of colors. Some techniques have been used since Roman times. The effects are normally permanent, and the treatment creates no special care requirements.

GEMOLOGY

• A mineral species closely related to quartz. In fact, mineralogists consider it a quartz variety. Composed mostly of silicon and oxygen: chemical formula SiO₂, which is the same as quartz.
• Agate, bloodstone, carnelian, onyx, and sardonyx are well known chalcedony varieties. Others include:

             Amethystine chalcedony – purple

             Chrysocolla chalcedony – bright blue or blue-green

             Chrysoprase – light yellowish green

             Plasma – dark green with white or yellow spots

             Prase – dark green

             Sard – dark orange, brownish red, or brown

CARE

Chalcedony generally has good wearability.

• Hardness: Moderate scratch resistance. Rates 6.5 to 7 on the Mohs Hardness Scale.
• Toughness: Good resistance to chipping and breaking.
• Stability: If dyed, color may eventually fade.
• Cleaning: Liquid cleaner, or detergent and water. Ultrasonic is usually safe.