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

Stone information is reference from

Gem Reference Guide, Published by GIA. ISBN 0-87311-019-6

Gemstones of the World, Published by Sterling Publishing Co., Inc NY, ISBN 1-4027-4016-6

Encyclopedia of GEMSTONE, Published by ISHINSHA CO., LTD. ISBN 978-986-251-831-1