Kyanite

Kyanite forms under high-pressure metamorphic conditions - specifically when aluminum-rich sedimentary rocks (like shale or mudstone) are subjected to intense pressure during tectonic events. It's one of three polymorphs of aluminum silicate (along with andalusite and sillimanite), each of which forms under different pressure-temperature conditions. Kyanite is the high-pressure member.

This makes kyanite a valuable indicator mineral for geologists - finding kyanite in a rock tells them it experienced pressures equivalent to burial depths of 15+ kilometers. The crystal's blade-like habit (long, flat crystals) is distinctive and immediately recognizable.

Kyanite has one of the most unusual physical properties in mineralogy: its hardness varies depending on direction. Along the length of the blade, hardness is about 4.5 (scratchable with a knife). Across the width, hardness jumps to 6.5-7 (not scratchable with a knife). This directional hardness variation (called anisotropy) is extreme in kyanite and rarely seen so dramatically in other minerals.

Kyanite's blade-like crystal habit and blue color are its most distinctive features. The directional hardness is diagnostic - try scratching along the length versus across the width. If the knife scratches one direction but not the other, it's almost certainly kyanite.

Distinguish from blue tourmaline (round cross-section, striated), sodalite (massive, no blade habit), and dumortierite (fibrous, not bladed). Kyanite crystals often show color variation along their length, with deeper blue at the center fading to white or clear at the edges. The pearly luster on cleavage surfaces is also characteristic.