Black Onyx

chalcedony (kal-SED-uh-nee) is microcrystalline or cryptocrystalline quartz; hexagonal (trigonal) crystal system; chemical composition SiO2
semitransparent to opaque
solid black, no banded, milky chalcedony with dyed treatment

Identification

Optic Character: AGG
Refractive Index: 1.535-1.539; 1.53 or 1.54 spot reading
Birefringence: usually not detectable, but may show .004
Pleochroism: none
Ultraviolet Fluorescence: generally inert (LW and SW); some may fluoresce a yellowish green (LW and SW)
Absorption Spectra: dyed green – vague lines around 645 and 670 nm
Specific Gravity: 2.60 (+.10,-.05)
Polish Luster: greasy to vitreous
Fracture: conchoidal; sometimes granular
Luster: dull to waxy
Cleavage: none
Identifying Characteristics: dyed green and dyed blue appear red or pinkish through Chelsea filter
Crystal Habit: aggregate, often in botryoidal forms
Hardness:6 ½ to 7
Toughness: good

Enhancements

Method: dyeing

Effect: chalcedony is dyed to practically all colors for various uses; the bands in agate and Onyx accept dye differently so that a green dye, for example, may impart color to some layers and leave others white.
Prevalence: common
Detection: appearance (color often looks unnatural), spectrum, Chelsea filter reaction, SG (dyed green often lower (2.60 or less) than chrysoprase)
Stability: generally stable
Special Care Instructions: none

Sources: Brazil, Uruguay