sentence: Obsidian has little water,  , but takes it in if it sits in groundwater.
sentences_before: Obsidian does not last forever. Over time, the glass becomes fine-grained crystals. So, no obsidian is older than Cretaceous age. It breaks down faster when water is present.
concept_name: Obsidian
headings: Obsidian Origin and properties
categories: Gemstones Igneous rocks