Onyx Marble | Gemopedia

The term onyx marble is a misnomer sometimes used to describe a rock made up of alternating layers of different colored calcite. The alternating bands closely resemble those that can form in onyx, however, onyx is a silicate mineral and calcite is a carbonate and their physical properties are dramatically different.
Alternate
Names
Marble Onyx, Banded Calcite
Colors
Various

Onyx Marble Classification

Common Name

Onyx Marble

Species

Calcite

Variety

Onyx Marble

Colors

Various

Alternate Names

Marble Onyx, Banded Calcite

Gemstone Groups

Calcite

Key Separations

cleavage, refractive index, birefringence (using distant vision method), specific gravity. Use birefrigence blink method.

Comments

When banded and Translucent to opaque, the calcite variety is referred to as "onyx marble," although some disfavor this nomenclature. Misnomers: Oriental alabaster, Mexican onyx and California onyx. Type of limestone used for ornamental objects.

Onyx Marble Optical Properties

Transparency

Semitranslucent - Opaque

Refractive Index

1.486-1.658

Birefringence

0.172-0.172

Optic Character

Uniaxial

Optic Sign

Negative

Polariscope Reaction

Aggregate (AGG)

Fluorescence

SWUV: Variable
LWUV: Variable

CCF Reaction

Pleochroism

None

Dispersion

Comments

Onyx Marble Chemistry & Crystallography

Chemical Name

calcium carbonate

Chemical Formula

CaCO3

Synthesis

Crystal System

Trigonal

Classification

Carbonate

Nature

Natural

Crystallinity

Aggregate

Comments

Onyx Marble Characteristic Physical Properties

Hardness

3

Streak

Varies By Color

Specific Gravity

2.65-2.75 Range:+/-0.05 Typical:2.7

Toughness

Varies

Inclusions

Onyx marble is a banded granular aggregate and is often dyed.

Luster

Vitreous

Stability

Fair

Fracture

Uneven, Granular, Splintery

Cleavage

Perfect, in three directions

Comments

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