Assignment 20 - Quartz & Calcedony Flashcards
A cryptocrystalline quartz aggregate.
Chalcedony
Chalcedony with curved or angular bands or layers that differ in color and transparency.
Agate
What makes amethyst the most-prized quartz variety?
Its wide availability, affordable price, and attractive color make amethyst one of the most popular colored gemstones.
Which treatment produces most of the citrine on the market?
Most citrine on the market is produced by heat treatment of amethyst.
Why is synthetic quartz used so widely in industry?
Synthetic quartz’s ability to vibrate at a constant frequency and its freedom from twinning make it ideal for use in electronics.
What are the four quality grades of amethyst?
The first grade includes the best amethysts. They’re a vivid, medium-dark to dark reddish purple or purple, and the best of them have no visible face-up color zoning. This grade might be described as “super,” “extra,” or “AAA.” Some dealers also use “African” or “Zambian” for this grade.
Gems in the second grade are a little less saturated in color. Although they’re attractive, some might show face-up color zoning. Or they might be slightly included. Dealers often call calibrated amethysts in this grade “medium-dark,” “AA” or “A.”
The third grade is considerably lighter in tone and saturation than the better ones—a light desaturated purple, known in the trade as lilac, rather than an intense purple. Gems in this grade are popular in mass-market jewelry, and they’re usually much less expensive than more intensely colored goods. Dealers might call them “B” or “medium.”
The fourth and lowest grade is lighter still, and not very attractive. Hues are less saturated, so they take on a grayish to slightly grayish pale purple color. Stones with a brownish tint are even less marketable. Gems in this grade tend to be low in value. Dealers often call them “light” or “C.”
Transparent quartz, usually rock crystal, that contains eye-visible inclusions, often named according to the type of inclusions it contains.
Sagenitic quartz
A type of sagenitic quartz containing needle-like rutile inclusions
Rutilated quartz
A type of sagenitic quartz containing needle- like or rod-like tourmaline inclusions.
Tourmalinated quartz
A property of some crystals that causes them to generate an electrical charge when subjected to mechanical stress.
Piezoelectricity
What’s the most valuable chalcedony variety?
Chrystoprase is the most prized of all fine-quality chalcedonies.
Chrysocolla-in-chalcedony is one of the most valuable chalcedony varieties.
What are agate’s distinguishing characteristics?
Agate’s bands can be distinguished from those of sardonyx or jasper because they’re curved or angular, not straight. The stripes are shaped that way because agate usually forms inside gas cavities in volcanic rocks, and its growth bands follow the contours of the cavities.
How do treatments make chalcedony more marketable?
Chemically treated black chalcedony—known in the trade as black onyx—is sold in larger quantities than any other chalcedony variety.
A crystal growth habit with a bumpy appearance similar to a bunch of grapes.
Botryoidal
A tablet-cut gem with a flat or slightly domed top and sometimes a carved design, typically set in men’s rings and cufflinks.
Signet
What are the three groups of varieties of Quartz according to crystal structure?
- Large single or twinned crystals
- Microcrystalline aggregates of small, but not microscopic, crystals
- Cryptocrystalline aggregates of tiny microscopic crystals
