A mineral form of aluminum silicate, typically blue in color; another name for kyanite.
From Greek dis- (twice) + sthenos (strength). Named for the mineral's two-directional hardness—it's harder when scratched parallel to its length than perpendicular to it.
Geologists named this mineral for a property that seems almost contradictory: it's simultaneously hard and soft depending on direction, making it useful for studying how crystalline materials resist force.
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