This ruby pebble, a rock-shop specimen from India, displays the clean hexagonal cross-section of corundum crystals. The flat face on this side is a parting plane, a break that results from a crystal weakness, in this case a plane of twinning. Corundum is a fairly heavy mineral, but it is extremely hard (hardness 9 on the Mohs scale) and can occur in streambeds as placer deposits, like the famous gem gravels of Sri Lanka.
The finest gem ruby stones have a red-purplish color called pigeon's blood. I've never bled a pigeon, but I think that's what this color is.
Ruby owes its red color to chromium impurities. The green mica accompanying this ruby specimen is fuchsite, a chromium-rich variety of muscovite.