A stretch bracelet in 8 mm silver sheen obsidian beads, dark grey to black with a metallic shimmer that moves as you turn your wrist.
The sheen is gas. Obsidian forms when silica-rich lava cools too fast for its atoms to organise into crystals, so it sets as glass. Tiny bubbles trapped in that glass sit in roughly parallel layers, and light bouncing off them produces the silver sheen. Turn the bead and the sheen moves, because a different layer catches the light.
It is glass, which means it is brittle. Harder than fluorite at 5 to 5.5, but a sharp knock will chip a bead rather than scratch it, and a chipped obsidian edge is genuinely sharp.
Mineral: Obsidian, a volcanic glass
Crystal system: None, obsidian is amorphous
Origin: Not recorded
Hardness: 5 to 5.5 on the Mohs scale; brittle, chips rather than scratches
Treatment: None
Approx. inner circumference: 7 cm unstretched
Bead diameter: 8 mm
Approx. weight: 15 g
Stringing: Elastic, stretch fit
Photographed as an example. These are strung in batches from the same material, so yours will be closely similar in colour and pattern but not the exact bracelet pictured.