It is aquatic sand from one of my tropical tanks, and is black the white specks are the grains that have been turned. I had to use a modified engineering tool to turn the sand as it blunted the wood turning tools I have. I bonded the sand to the tubes with an industrial adhesive similar to CA but stronger, then polished with a glass elastic wheel and then a glass polish ( I used to do a lot of glass engraving ). I just wanted to try something different that I hadn't seen before. I love one offs so pen turning really appeals to me, and I feel you are only limited by what you can imagine.
Thanks to everyone for the kind words of encouragment