The chemistry, properties and tests of precious stones
Chapter 15
BLACK STONES.
_Hardness._ _Specific Gravity._ (See Chapter VII.) (See Chapter VIII.) Diamond 10 3.502-3.564 Garnet 6-1/2-7-1/2 3.400-4.500 Jet 3-1/2 1.348 Onyx (a variety of Chalcedony) 6-1/2 2.598-2.610 Quartz 7 2.670 Tourmaline (not unlike Black Resin in appearance) 7-1/4 3.024-3.300