How do you tell driference black jade or black onyx?

Answer 1

The density of onyx is 2.65 g/cc or kg/m^3

The density of jade ranges from 2.9 to 3.05 g/cc

With a decent set of letter scales capable of measuring to 0.1 g, and a syringe capable of measuring 0.1 cc one should easily be able to make density measurements a la Archimedes to a sufficient accuracy for any masses of black onyx or jade above ~1.5 g.

In short, weigh your black stuff.

Next get a measured volume of water, put your black stuff in the water and measure the volume of displaced water.

The weight divided by the volume is the density.