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: Onyx & Sardonyx

Black Onyx, Sard, & Sardonyx used in Jewelry

Source: Occuring worldwide

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

Onyx and sardonyx are a translucent to opaque variety of chalcedony and agate, which are a microcrystalline or cryptocrystalline form of quartz, who's main chemical composition is silicon dioxide. Onyx, sard and sardonyx form from the deposition of silica which is carried by water that percolates through fissures within solid rock.

The name "onyx" is derived from the Greek word meaning "veined gem," which only applies to the banded agate variety of onyx. Solid black onyx is totally opaque and typically cut en cabochon, or shaped into beads. 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," which was the stone that comprised the Ephod, an oracular object in ancient Israelite culture that represented the tribe of Joseph.

Black onyx and gold necklace (Michael Borofka)

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

The name "sardonyx" refers to a type of agate/sard/onyx that is banded, or stratified with alternating layers of brownish-red to pink sard and milk-white chalcedony. If the sard is replaced by red carnelian it is referred to as "carnelian onyx." The name "sardonyx" is derived from the Greek, word sard, meaning "reddish brown" in color. The use of sardonyx in the carving of cameos or intaglios was popularized by the ancient Greeks. Most contemporary sardonyx is cut from South American agate.

Roman Sardonyx Cameo (photo: public domain)

Victorian Sardonyx cameo

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

Diaphaneity translucent, subtranslucent, opaque

Gem Color black, grey

There is an imitation sardonyx that is made by cementing together two correctly colored stones, or by coating sard with a layer of sodium carbonate, then placing the stone on a hot iron to create the milk-white layer.

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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