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