Sapphire
Sapphire is the gem variety of corundum (all non-red gem corundum is called sapphire). Montana is the only significant US source — and it produces two distinct types: Yogo sapphires from Yogo Gulch are a rare cornflower blue found nowhere else on Earth; alluvial sapphires from the Missouri River and Gem Mountain come in a rainbow of colors and can be mined by the public.
Physical Properties
Chemical Formula
Al₂O₃ (corundum)
Hardness (Mohs)
9
Color
Blue (most famous), yellow, orange, green, purple, pink, colorless
Luster
Vitreous to adamantine
Crystal System
Hexagonal (trigonal)
Streak
White
Cleavage
None (parting only)
Specific Gravity
3.99–4.1
🔍 Field Identification Tips
Hardness 9 (second hardest natural mineral, only diamond is harder). No cleavage — very tough. Hexagonal crystals often barrel-shaped. Montana sapphires are typically small (under 1 ct rough) but exceptional quality. Distinguished from tanzanite (softer) and iolite (softer) by extreme hardness.
Varieties & Forms
📍 Where to Find Sapphire in the US
Uses & Applications
Fine jewelry (September birthstone), industrial uses (watch crystals, scientific instruments), LED substrates.
💰 Collector Value Guide
Montana rough: $5–$200/ct. Cut Yogo sapphires: $500–$5,000+/ct. Heat-treated commercial sapphires: $10–$100/ct.
Find Sapphire With Our Tools
Use the We Lick Rocks app to identify specimens, find dig sites, and connect with local rockhounds.