Simplify the following text for a Grade 8 native English-speaking person.
Original: Because obsidian is metastable at the Earth 's surface ( over time the glass becomes fine-grained mineral crystals ) , no obsidian has been found that is older than Cretaceous age .
Simplified: Obsidian is metastable at the earth 's surface : over time the glass becomes fine-grained crystals . So , no obsidian is older than Cretaceous age .