CS Book 4_AS-20 Quartz:Chalcedony PDF

Quartz and Chalcedony KEY CONCEPTS Quartz varieties differ mostly in the size of their crystals. The finest citrine color is a saturated yellow to reddish orange,...

Quartz and Chalcedony KEY CONCEPTS Quartz varieties differ mostly in the size of their crystals. The finest citrine color is a saturated yellow to reddish orange, free of brownish tints. Many gem-quality quartz varieties are routinely heated, bleached, or dyed. Most citrine on the market is produced by heat treatment of amethyst. The gem trade considers chalcedony a separate species from large-crystal quartz varieties. Because smoky quartz crystals are often large, inexpensive, and free from flaws, they’re ideal for gem carvers and sculptors. Its wide availability, affordable price, and attractive color make amethyst one of the most popular colored gemstones. One of the first gems to be fashioned, chalcedony has been used in jewelry for almost 3,000 years. Varied color distribution within amethyst rough yields fashioned gems in a wide variety of tones and saturations. The finest chrysoprase color is a light to medium yellowish green. The finest amethysts are a strongly saturated, medium-dark to dark reddish purple or purple, with no visible color zoning. Chrysocolla-in-chalcedony is one of the most valuable chalcedony varieties. Amethyst is sold in a variety of grades based mainly on color. Treated black chalcedony—known in the trade as black onyx— Zambian amethyst often retains its deep color even in small sizes. is sold in larger quantities than any other chalcedony variety. Brazil is the world’s major source of commercial-quality Collectors often give agates descriptive names based on their amethyst. appearance. Key Terms Agate—Chalcedony with curved or angular bands or Sagenitic quartz—Transparent quartz, usually rock crystal, layers that differ in color and transparency. that contains eye-visible inclusions, often named according to the type of inclusions it contains. Botryoidal—A crystal growth habit with a bumpy appearance similar to a bunch of grapes. Signet—A tablet-cut gem with a flat or slightly domed top and sometimes a carved design, typically set in men’s Chalcedony—A cryptocrystalline quartz aggregate. rings and cufflinks. Piezoelectricity—A property of some crystals that causes Tourmalinated quartz—A type of sagenitic quartz them to generate an electrical charge when subjected to containing needle-like or rod-like tourmaline inclusions. mechanical stress. Rutilated quartz—A type of sagenitic quartz containing needle-like rutile inclusions 285