Kidney Stone

Kidney stone is used for various materials, an is best avoided:
a. A nodule of iron stone common in the Oxford clay (Middle Oolite) of England.
b. A tough compact fine grained greenish or bluish amphibole; same as nephrite. Standard, 1964
c. A kidney-shaped pebble. Webster, 3d.
d. Kidney stones are most commonly thought of as those very painful little stones generated in the kidneys under certain conditions. The most common minerals were, not surprisingly, phosphates. These include apatite, brushite and whitlockite. Apatite is the most common mineral in many kidney stones, forming crumbly to solid white, yellow or brownish masses. Various forms of apatite were interlayered, like the layers in hailstones. Brushite occurs as tabular to bladed yellow to white crystals typical of kidney stones formed under more acidic conditions. Whitlockite forms amber to brown coatings on some stones, and is particularly common in prostatic stones.
Two calcium oxalates, whewellite and weddellite are abundant in kidney stones. Outside the body, these minerals are rare, found most often on the deep sea floor, in coal seams and in sedimentary nodules. In kidney stones the whewillite forms globular to radiating masses of crystals while weddellite forms sharp dipyramidal crystals up to 5 mm long (ouch).
Magnesium phosphates, such as struvite and newberryite are rare minerals generally found ouside the human body only in bat guano. They are apparently deposited in kidney stones by particular bacterial infections. Struvite forms colorless crystals lining cracks in the stones formed under alkaline conditions. Newberyite forms pale green to white spherules on the surface of some stones.
Some minerals found in kidney stones are more familiar to rockhounds. Calcite and aragonite are rare as granular material intergrown with the phosphates in kidney stones. Stones from the human pancreas are often calcite. Halite was found a few times and could be a contaminant from salty fluids in which the stones are stored during shipping. Gypsum was found three times as white crystals encrusting the oxalates. (from Gibson, Dick, 1974, "Descriptive Human Pathological Mineralogy", American Mineralogist, vol. 59, p. 1177-1182, via Bill Cordua, Uni Wisconsin.)