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: Onyx

Black Onyx used in Jewelry

Source: Occuring worldwide

Birthstone: Leo: July 23 - Aug 22 (February alternate to amethyst)

Onyx (aka 'sardonyx' or carnelian) is a translucent to opaque variety of chalcedony, a microcrystalline or cryptocrystalline form of quartz, who's chemical composition is silicon dioxide. Onyx forms from the deposition of silica carried by water percolating through fissures within solid rock.

The name "Onyx" is derived from the Greek word meaning "veined gem," and only applies the the agate variety. The name "Sardonyx" is derived from the Greek, word "Sard" meaning "reddish brown," applying to the brown, and white banded variety. The name "Carnelian" refers to a red and white banded variety of agate.

Black onyx and gold necklace (Michael Borofka)

Onyx pendant (Ray Tracey), earrings (Jeff & Susan Wise)

Onyx is totally opaque and usually cut as a cabochon, or into beads, and is also used for intaglios and cameos, where the bands make the image contrast with the background. Onyx is the birthstone of Leo: July 23 - Aug 22. According to the 13th century rabbi Rebbenu Bachya, the word Shoham in the verse Exodus 28:20 means "Onyx" and was the stone on the Ephod representing the tribe of Joseph.

Onyx belongs to the tectosilicate quartz family of minerals that include chalcedony, carnelian' or 'sard,' agate, and jasper. Onyx is typically associated with igneous rock, grown in pegmatites and geodes that formed during the mountain-building process.

Onyx (Microcrystalline Quartz) Chemistry & Physical Properties

Crystal System	microcrystalline
Crystal Habit	cryptocrystalline
Specific gravity (SG)	2.58 - 2.61
Mohs Hardness Scale	7
Toughness	fair to good
Fracture	conchoidal, uneven, splintery
Cleavage	none
Streak	white
Chemical Composition	SiO₂ (Quartz)

Onyx (Microcrystalline Quartz) Optical Properties

Optical Properties	singly refractive
Refractive Index	1.486
Birefringence	+0.009
Pleochroism	none
Surface Luster	vitreous, dull, waxy
Diaphaniety	translucent, subtranslucent, opaque
Gem Color	black, grey

Bibliography and Reference on Onyx

1. Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.

2. A.C. Akhavan, Twinning in Quartz Crystals www.quartzpage.de

3. GIA, Characteristics of Citrine, Amethyst & Smoky Quartz www.gia.edu

4. Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition

5. Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy . Gemstone Press

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