Something that I found to be quite interesting and hope you do as well.
Many assumed that all stones that are black, shiny and not plastic were onyx. Well, how amazingly wrong one can be. I discovered blackstone about a year and a half ago, but only recently learned how to unequivocally know the difference between onyx and blackstone when purchasing beads. Historically I had insisted upon purchasing Onyx simply because I knew what I was buying. Thankfully, I have been purchasing my supplies from the best bead dealer in Virginia because she took time to explain the differences! (More on my amazing vendors in a future post!)
You can tell the difference in the stones by the marks surrounding the drilled holes in the sides of the beads. If the area surrounding the drilled hole is completely black it is Onyx. If it is lighter around the drilled hole, making the drilled area appear almost gray the bead is blackstone.
As with any natural stone, it is rare that the holes drilled in the beads will all be uniform. If you receive a string of beads that are completely uniform in color, with perfectly round uniform holes, you might want to take a second look. Either the quality is amazing, or you might have plastic. Never be afraid to ask questions of the person you are buying beads from. It is an excellent way to be sure of what you are buying and an even better way to learn about stones!