What substance is described in the excerpt from Fast Facts for Free Agents on page 90?

The excerpt from Fast Facts for Free Agents describes a perplexing material that Leopard People encounter. It is a rare substance that is even more unbreakable than a diamond. It is often found embedded in ceremonial rings and sometimes is found in the blade of a juju knife. The hard, clear green substance is so rare that it has no name and no one knows where it comes from.