QUARTZ
SMOKY QUARTZ
ROCK CRYSTAL
CITRINE
ROSE QUARTZ
AMETRINE
AMETHYST
- Quartz varieties differ mostly in the size of their crystals
- Same chemical elements: Silicon & Oxygen
- Crystallize in the trigonal crystal system
- Most gem-quality quartz varieties are routinely heated, bleached, or dyed.
- The gem trade considers chalcedony a separate species from large-crystal quartz varieties.
- Gem varieties of quartz are abundant
- Most commercially important gem-quality quartz
- Its wide availability, affordable price & attractive color makes it one of most popular colored stone
Quality
- Natural irradiation acting on trace elements of iron in its crystal structure
- Varied color distribution in a wide variety of tones & saturations
Finest Quality
Strongly saturated, medium-dark to dark reddish purple or purple, w/ no visible color zoning
Lower Value: Brownish or bronze-colored tines or any noticeable color zoning
Color Zoning - Most visible to view face down through its pavilion on a white background
Market & Sources
- Africa is the major source of the finest-quality amethyst.
- Brazil is the world’s major source of commercial-quality amethyst.
Sourth America
- as a prolific source of lighter- colored goods.
- Accounts for 75% of all calibrated, commercial-quality amethysts on the market
Zambia & Brazil - 2 of the most important amethyst sources.
Grading
First Grade: no face-up color zoning, vivid, medium- dark to dark reddish purple or purple
"AAA", "Super","Extra", “African” or “Zambian”
Second Grade: might show face-up color zoning, a little less saturated in color
“medium-dark,” “AA” or “A.”
Third Grade: lighter in tone & saturation than the better ones—a light lilac rather than an intense purple. “B” or “medium.”
Fourth Grade: light & no very attractive, grayish to slighly grayish pale purple, low in value
“C” or “Light”
Dealers often characterize amethyst from Brazil and Uruguay as pale, but both sources can produce gems comparable to African material.
Heat Treatment
Finest citrine: Saturated yellow to reddish orange, free of brownish tints.
Since natural citrine is rare, most citrine on the market is produced by heat treatment of amethyst.
Top sources for natural citrine are Bolivia, Spain, Madagascar, Mexico, and Uruguay.
Finest Ametrine: a combination of medium-dark to moderately strong orange & vivid to strong purple or violetish purple
-- attractive half-and-half distribution of each color, with a sharp boundary
Often large, inexpensive, and free from flaws, they’re ideal for gem carvers and sculptors.
Amethyst cutting & marketing centers - Germany, Thailand, China, and India
- Lighten the color of dark stone
- Remove undesirable brownish hues
Synthetic
- First commercially synthesized in 1950s
- Hydrothermal method produced stone in 1970s
- Price drops because of a plunge in buyer confidence
Trace Element: Iron
Attractive Alternative for
Topaz & Yellow Sapphire
Often cut as a rectangular step cut because it displays the bicolor effect nicely.
Only one natural source - Bolivia’s Anahi mine
Heat treatment & Irradiation with starting material like color-zoned amethyst
Morion: very dark brown smoky quartz
Finest Smoky Quartz: Deep reddish brown that's been compared to cherry wood
The color is from natural radiation. The radiation creates color centers that involve aluminum impurities
Heat treatment --> changes to greenish yellow that resembles citrine
CHATOYANT QUARTZES
Have internal fractures --> a cloudy translucence
Finest Rose Quartz: Highly transparent & usually faceted, typically comes from Brazil
Show Asterism --> when rose quartz contains numerous small inclusions
Occasionally, it's treated w/ irradiation to intensify its color
Faceted rock crystal --> used for diamond simulant
Trade name for transparent,
bicolored variety of quartz
A Trade name for colorless,
transparent quartz
Eye-visible inclusions can contribute to the gem's beauty
- Sagenitic quartz: eye-visible needle-like mineral inclusions
- Rutilated quartz : A type of sagenitic quartz containing needle-like rutile inclusions.
- Tourmalinated quartz: A type
of sagenitic quartz containing needle-like or rod-like tourmaline inclusions.
Important rock crystal sources: Brazil, Madagascar, Switzerland, and the United States.
Cat's-eye quartz
- a semitransparent to translucent brownish yellow, brownish green, or greenish yellow
- Best quality can resemble cat's-eye chrysoberyl
Tiger's-eye (microcrystalline aggrerates)
- a semi-transparent to opaque gem has a pattern of lustrous yellow & brown bands that reverse color when the stone is turned
- Commonly heated to produce a reddish brown color
Zebra Tiger's-eye: A variegated grayish blue & brown variety of tiger's-eye
Hawk's-eye(microcrystalline aggrerates): grayish blue color
Sources: India, Sri Lanka, South Africa, Myanmar & Western Australia
QUARTZITE & AVENTURINE QUARTZ
Aventurine Quartz - Translucent to opaque, it glitters with aventurescence caused by light reflecting from small inclusions of mica or other minerals.
Sources: India (Major), Brazil, Russia & Tanzania
Quartzite - Commonly dyed to produce intensely colored, inexpensive gemstones, most often dyed green to resemble jade
Maraba amethyst is more uniform in color & high clarity