Because obsidian is metastable at the LOCATION@1 's surface -LRB- over time the glass becomes fine-grained mineral crystals -RRB- , no obsidian has been found that is older than MISC@1 age .

Because obsidian is metastable at the earth 's surface -LRB- over time the glass becomes fine-grained mineral crystals -RRB- , no obsidian has been found that is older than MISC@1 age .
