How to Cut Obsidian

Authocunknown
GOLD SHEEN: To get the most out of mahogany gold sheen obsidian, saw with the bands, as if they were a stack of plates, and you wish to unstuck them. Watch for "fire spots" in gold sheen. It is not plentiful, but opal like colors do sometimes occur in mahogany gold sheen.
IRIDESCENT: There are two types of iridescent obsidian. In cutting both correctly, the orientation of the color is most important. One type of obsidian is banded and the color lies in the bands. On the unbanded types of obsidian the surface has to be chipped to find the color. The banded type will have several colors or shades, while the unbanded types will have only one color. Cut the banded material parallel to the bands to get effect. To get a rainbow effect, cut the stone at an approximately 15-degree angle across the bands.
MIDNIGHT LACE: Lace-patterned obsidian should be cut across the surface pattern that you desire to reproduce. Although obsidian is comparatively soft, it is still very important to sand away all scratches before going to polish. Some advise that wet sanding be done, since obsidian is heat sensitive and very brittle. For final polish, felt with cerium oxide is the choice. Should you be faceting some particularly gemmy obsidian try cerium oxide on Lucite@, but keep it wet.
RAINBOW OBSIDIAN: Cut parallel to flow layers. These can be seen by examining fractured surfaces using an overhead single lamp bulb. As [the layers] are not always straight, it may be necessary to turn the stone slightly in the saw. Examine each slab set with either water or saw oil to see if the correct angle has been obtained.
SAFETY TIP: After obsidian is sawed, be sure to bevel the
edges all around on your fine grinding wheel to keep them from flaking and chipping. Wear goggles or glasses at all times. If a small chip of glass ( obsidian) got into your eye, it would be hard to remove as it is transparent and hard to see even with a powerful magnifying glass, and the edges may cut your eye to a great extent before it could be removed.
GRINDING OBSIDIAN CABS: Approach yourgrinding wheel with the material at a slight horizontal angle. If brought straight in, it may be a "shattering" experience, as obsidian fractures conchoidally and this is a sure way to do it.
POLISH ON OBSIDIAN: Keep the polishing wheel wet. A dry polishing will result in blisters and scratches. Obsidian is relatively inexpensive, easy to obtain and soft. With proper understanding of its glassy properties you can obtain some beautiful results.
-from Golden Frog, February 2005 Via The Rock Collector October, 2009 Via Strata Gem, November 2009