Aragonite is the orthorhombic polymorph of calcium carbonate (calcite is hexagonal).  Aragonite is stable at higher pressures than is calcite and commonly occurs in high-pressure, low-temperature blueschist-facies metamorphic rocks.  It also forms metastably in the shells of some marine organisms and as needles precipitated from warm tropical sea water, which can accrete to form ooids.  Like other carbonate minerals (e.g., calcite and dolomite), it has extreme birefringence and its relief changes with orientation.  Compare the views in image A and B (rotated 90 degrees to one another), in which aragonite occurs with acicular bundles of jadeite. As an orthorhombic mineral (with a 2V of 18 degrees), aragonite does not have the rhombohedral cleavage of calcite and dolomite, and only has imperfect {010} cleavage, evident in several grains in C.  One grain near the centre of image C has its cleavage approximately E-W and appears extinct under crossed nicols (D). Most of the other grains of aragonite in this image illustrate the high-order white interference colour typical of carbonate minerals.  Sample is of an aragonite-jadeite vein in a glaucophane schist from Pacheco Pass, California.  Views A and B are 2.5 mm across and C and D are 1.2 mm across.  A, B, C ppl, D x-nicols.