Thanks for posing such an interesting truly interdisciplinary question. I will try to answer as many aspects of your question as possible.

Obsidian is a volcanic mineral and not indefinitely stable over time. It forms at the surface of volcanic eruptions and is amorphous. This means it is not a rock with a defined crystal structure. Obsidian consists to more than 70% of SiO₂ (silicon dioxide), it is very similar to normal glass or quartz (a more crystalline form of SiO₂). A lot of organisms have a skeleton made up of SiO₂, e.g. some marine sponges. Obsidian is not white or transparent (like glass) because some metal impurities get trapped in the SiO₂ matrix during its formation. Predominantly these other metals are iron and magnesium.

SiO₂ is a very inert material, experts don't agree if Si is one of the essential elements we need or not. I don't think it is toxic in itself. Obsidian is used in cardiac surgery according to the information I have. Most infections due to glass related injuries result from some contamination of the glass surface, rather than the glass itself. Glass, over time, gets expelled from the wound if the pieces are small enough.

Obsidian is a lot softer than stainless steel. Using Mohs' scale of mineral hardness, obsidian would be 5 - 5.5, compared to stainless steel > 7 (diamond = 10). And stainless steel is a lot more durable, the breakdown of obsidian is accelerated by the presence of water. Obsidian might therefore not be a good replacement for stainless steel depending on your exact applications.

Thanks again for contacting us. I hope this answer will help you to make a decision given your problem.