Onyx

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A layered variety of chalcedony banded in parallel black and white, onyx also lends its name to uniformly black chalcedony when used as a gem. It forms in the cavities of volcanic rocks and in sedimentary silica deposits, most notably in India, Brazil, Uruguay, and the United States. Belonging to the trigonal quartz group, it measures 6.5 to 7.0 on the Mohs scale. Since antiquity the stone's contrasting layers have been exploited for cameos, intaglios, beads, and large decorative objects, while pure black onyx became popular in mourning and men's jewelry. A moderately priced semi-precious stone, onyx is traditionally associated with February.

Physical & Optical Properties

Category Semi-Precious Gems

Mohs Hardness 6.5 – 7.0

Specific Gravity 2.55 – 2.7

Refractive Index 1.53 – 1.543

Crystal System Trigonal

Color black

Mohs Hardness

Mohs hardness of Onyx shown against the ten reference minerals, from talc (1) to diamond (10).

Mohs	Reference mineral	Onyx
1	Talc
2	Gypsum
3	Calcite
4	Fluorite
5	Apatite
6	Orthoclase
7	Quartz	●
8	Topaz
9	Corundum
10	Diamond

Crystal System

Value & Market

Price Tier Moderate ($50-500/ct)

Where to Buy Onyx

Blue Nile Fine jewelry retailer

James Allen Gemstone jewelry

Brilliant Earth Ethically sourced gems

Amazon Wide selection

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Cultural Significance

Birthstone Month 2 (alternative)

Treatments

Black chalcedony ("onyx") is assumed to be dyed - Blue Chart Gem Identification, Herve Nicolas Lazzarelli, 2010, p 7 6.5 to 7Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references 2.55 to 2.70Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) 1.530 to 1.543Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) Uniaxial/+Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004) Straight color bands (black with white layer)Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references OpaqueHerve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references TrigonalUlrich Henn and Claudio C. Milisenda, Gemmological Tables (2004) Onyx information at mindat.org

About Onyx

オニキス（縞瑪瑙・玉髄）は、緻密な黒色の縞状カルセドニーで、一般に白い縞との組み合わせで知られます。古代ギリシャ・ローマ・メソポタミアから宝石印章・カメオ・印鑑彫刻の素材として珍重され、ブラジル・インド・ウルグアイが主要産地です。現在市場に出回るほとんどの「黒オニキス」は、シリカに浸透した糖液を酸で発色処理した染色品ですが、装飾価値は変わりません。