Czechoslovakian Black Stone Necklace

Hi, I have a "plastic?" necklace with a solid black faceted "stone" in a silver setting. The setting is marked "Made In Czechoslovakia" I have never come across a "stone" like this, and I was wondering if you could help.

The necklace was broken, and so I took one of the black faceted stones and it didn't feel like glass so I banged it very hard with a hammer. It didn't even leave a mark. Then, I thought it was plastic, so I held the bead under a flame. There are no marks, and a faint pleasant smell. It smelled nothing like plastic. The fire did not do any damage to the bead.

Could you please tell me what this bead might be made out of? It is the most curious and indestructible bead I have ever come across.

Thanks!


PS. I did not think they made plastic in Czechoslovakia, but maybe I am wrong.