Obsidian comes in two distinct forms; ejecta and ground material. Unlike most gem material it’s not a form of crystalline mineral at all, it is a naturally formed volcanic glass.
Silica is melted deep in the bowels of the fissure until it fuses into a flowing, liquid mass. Since it is less dense than the surrounding rocky materials, it floats to the surface. On top, it is first out during an eruption. It flies from the earth high up into the atmosphere. If the globules are large enough, and free from other material, it forms into droplets of molten glass and rains back down onto the surface. If the ejecta goes high enough, they cool as they fall. If they cool sufficiently, they form into spheres. If they land on sand or soil, they can survive the impact. If they arrive back on the surface in a malleable state, they deform into ovoids. Almost all of the material is contaminated with either ash or other minerals but there are a very few that are not, and they come down Clear. All of the others are coloured by the impurities, usually Black or Grey. This material has a local name in the southern US where the natives call it Apache Tears. It is easily differentiated from other material when viewed through the stone as it is a very dark, but transparent, species.
Otherwise, Obsidian forms in pockets and seams just like any other Igneous material. In the solidified magma flow where it is found in Black, Grey, Green or Brown, depending on what and how much, it is contaminated by other material. Most ground material is barely translucent to opaque.
Some Obsidian has sparkles, some has a sheen. Some has little white flowers and/or dots like Snowflake or Flower Obsidian. Some has fibrous needles inside. Some has stripes or spots of another colour, like the Brown in Mahogany Obsidian.
Obsidian has been valued through the ages for the fact that, as a glass with large conchoidial fractures, it is easily sharpened to an edge finer than a razor. In fact, modern surgeons have used Obsidian scalpels and found that they are keener, and maintain their edges longer, than metal units. The incisions created by Obsidian blades also healed better and quicker as there is no contamination from any metal contacting with the flesh. Of course, the big drawback is the fragility of the blade. Very few surgeons are willing to take the chance of leaving a flake of glass cutting away inside a patient if the blade were to fracture during the procedure. And the piece of blade is very difficult to see on an X-ray and is not magnetic. So most physicians have decided to leave the blades where they originated; back in the hands of the Inca priest and their human sacrifices.
We do not stock obsidian knives, but we do have pieces of this unusual material set in Sterling and loose as specimens. Please feel free to visit us to discuss your needs.