How to distinguish ebony, obsidian, black agate and black tourmaline? It is said that ebony will sink to the bottom, and obsidian has rainbow eyes. What about the latter two?

First of all, ebony and stone are best distinguished. Moreover, the density of black agate is not as good as that of black agate, which has a frosted and fragile feeling. Black agate can be tested by electrostatic hair, but it is not obvious and opaque, and the color may not be so uniform. Obsidian has a good luster, some have cat's eye effect, and the color is often accompanied by brown luster.