I have a new love hate relation ship in my shop with Obsidian. I basically refused to work with it as it is very problematic. It is volcanic glass and tends to often have tiny internal complicating issues, basically tiny bits of Pumice scattered inside the glass. It also tend to want to chip and that tendency is why it was so desirable for use as arrowheads. Metal grinding wheels tend to bring out this in the most dramatic and devastating way. In 3 of my previous attempts I ended up with either cracked scales or tiny "chips" along the bolster or spine. Well I seem to have figured out the chipping issue by only using foam backed wheels and I use a strong light to try and find the pumice inclusion that are big enough to show up which allows me to avoid most of them. Most of the really interesting Obsidians to use have some internal features that make it interesting and these form in bands in the liquid obsidian. I try to find a wide band and use a felt marker to mark the band around the piece so that I can attempt to cut the piece in line with the bands as much as is practical. This is not an exact science as the bands are not completely straight. The dip and rise in ways not obvious inside and through the piece. Here is a photo of a piece with shimmering silver bands. I used a thick mostly silver one as the band I want to track around the stone and I will glue a block of wood as close to in line with this band. Hopefully when I then saw the piece I will get and interesting swirling pattern across the slabs that would look interesting as knife scales.