Enter a clue or pattern or both:

The Clue

Relative of perlite (8 letter answer)

The Answer

OBSIDIAN

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Notes

The answer OBSIDIAN is so rare that it appears less than once every 2500 puzzles.

Definition

OBSIDIAN as a noun:

1. acid or granitic glass formed by the rapid cooling of lava without crystallization; usually dark, but transparent in thin pieces