calcite

Softer than a knife, and it fizzes hard in vinegar. A steel blade scratches it easily where quartz shrugs the blade off, and quartz never fizzes at all. Turn it in the light and the cleavage glints in little rhombic steps, flat faces meeting at slanted angles.

On the beach it turns up as white or honey-colored crystal filling vugs in limestone and in Keweenaw basalt. Soft, fizzing, glinting in rhombs: three checks, one answer.