Onyx - August's Alternate Birthstone

Onyx: comes in many forms, from sardonyx, to cameo's, to intaglio's, to cabochons, and many more forms.

Birthstone: August
Moh's hardness: 7
Specific Gravity: 2.65 - 2.66
Color: Black, white, green, brown, red
Chemical Composition: SiO2 - Silicon Dioxide
Refractive Index: 1.543 - 1.552 to 1.545 - 1.554

Onyx is a layer stone with the combination of a black base and white upper layer also called true onyx or Arabic onyx. In cabochons and beads, black and white layers may alternate. On the other hand, onyx is also a name sometimes used for unicolored chalcedony. This must not be confused with onyx marble, which in short is also called onyx. The name onyx has its origin in the Greek language and means "fingernail" probably because of its weak transparency.

Onyx Intaglio: name for engraving with negative image into that stone, as used for seals.

Onyx Cameo: name for a relief which is cut so that it is raised, usually the bottom layer being a dark color while the relief being carved out of the white layer on top.

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