noun
natural glass
Refers to glass formed by natural processes, such as obsidian or fulgurite, as opposed to man-made glass.
黒曜石は天然ガラスの一種です。
Obsidian is a type of natural glass.
Rare kanji spelling using 硝子 for glass; 天然ガラス is overwhelmingly more common.
ガラス alone usually refers to man-made glass; 天然ガラス specifies naturally occurring glass.
Compound of 天然 (natural) and ガラス (glass).