Kyanite comes in many colours, most commonly in blue or green, sometimes pink or yellow. It usually has icy needle-like inclusions, sometimes flecked with spots, which remind me of glaciers (which are not always clean anyway!). Kyanite crystals have an interesting property: different hardness values parallel (5) and perpendicular (7) to the cleavage. Gem-quality stones are rare, but when you can get hold of them, they sport an astonishingly clear shade of cornflower blue, it's like scoring high-end sapphires at a fraction of the cost :P

Here, I'm sure you can spot the opaque (but still pretty) gems right away: