How do I break up a lump of obsidian?

[naive]

I've very generously been given a lump of obsidian roughly the size of a lump of coal. As a hobbyist jeweller I'd love to break it up into smaller pieces to be set in silver and worn as a jewellery, but I'm not sure of the best way to break it up. Currently the hammer/chisel option is looking the most favourable but if anyone knows of a good method for cutting it, I'd love to hear about it! Thank you :)

[Teddio]

Obsidian is a volcanic glass and is very brittle, breaking with a conchoidal fracture when struck with a hammer. Your best bet is to use a fine diamond rotary saw.

[heathfield]

And wear thick leather gloves and goggles!