noun
volcanic glass
A geological term for natural glass formed by rapid cooling of lava, such as obsidian.
火山ガラスは、マグマが急に冷えてできる。
Volcanic glass forms when magma cools rapidly.
黒曜石 (obsidian) is a specific type of volcanic glass, while 火山ガラス is the broader category.
Compound of 火山 (volcano) and ガラス (glass), a direct calque of the English term 'volcanic glass'.