Physical Properties of Gold for Identification and Classification Purposes.
This site contains information on the Physical Properties and Chemical Composition of Minerals, gemstones, crystals, precious metals, and sedimentary, igneous and metamorphic rocks for crystallography, geology, identification of minerals, Jewelry and mineralogy. This includes cleavage, description of crystal formations, crystal structure, hardness, specific gravity, Mohs’ Scale, crystal features and crystal habits for identification and classification purposes.
General
Onyx ( SiO2 with impurities) is a member of the Quartz family
Chalcedony or microcrystalline quartz.
Color
Onyx is usually black, ray or brown with white bands. The bands are
typically straight, which separates it from banded agate, because the
lines in banded agate are usually curved. Sardonyx is a type of Onyx with
bands of Sard alternating with the stone’s white or black layers.
Imitators
Sometimes dishonest merchants will sell grayish chalcedony or agate that
has been dyed black or green to imitate Onyx. Calcite
is also sold as Onyx but discerning the two is actually quite easy because
Onyx is very hard, but something as soft as a fingernail can scratch
Calcite. Also, some marble has been called onyx, but is much softer
and subject to damage from acids.
Uses
Black Onyx is often used as a backdrop in jewelry, such as cameos. It is
also used as cabochons and as a building material.
Enhancements
Black and colored onyx is always dyed. Onyz is easily dyed, so beware of
the more "unnatural" looking colors, such as bright blue, etc.
Onyx with red and white bands is sometimes referred to as "Sardonyx".
Onyx is the black variety of agate. However some parts are more or less
translucent. This is why it is often heated to accentuate the intensity of
its color.
Specific Gravity 2.65
Refractive
Index 1.55 - 1.54
Hardness 6.5 - 7.0.
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