Ashes and Obsidian

Ashes noun - The portion or bits of something left over or behind after it has been destroyed.

Obsidian and ashes are semantically related In some cases you can use "Obsidian" instead a noun "Ashes".

Obsidian noun - Acid or granitic glass formed by the rapid cooling of lava without crystallization; usually dark, but transparent in thin pieces.

Ashes and obsidian are semantically related in volcano matter topic. You can use "Ashes" instead a noun "Obsidian".