Onyx Cave, Arkansas

Draperies of flowstone are only one of several types of speleothems, or cave formation, that can be found in Onyx Cave, near Eureka Springs, AR. The term 'onyx' is not a mineral name, but rather a commercial one. It has been used to refer to a number of different rocks or gemstones over history, but most often refers, today, to a dark or black agate or to travertine calcite. The onyx of Onyx Cave is travertine calcite, or limestone, and the formations were formed by slow precipitation of the mineral from groundwater.

Onyx cave is a publically accessible tour cave.  Several other caves that are open for tours are also found in the same region, and this isn't a coincidence.  The southern Ozark Plateau has one of the densest populations of caves and karst formations on the planet.  The region is underpinned by up to 1500 feet of soluble limestone and dolostone, and  is very old.  Most of the sedimentary rocks are 340 million to half a billion years old, and they have been heavily fractured at several points in geologic history.  Great age, soluble rock, fractures, and groundwater means caves - lots of caves.

Malachite

This is probably the cutest malachite cabochon I've ever seen, but how the face got there is even more interesting than how adorably happy it looks.  The interesting banding and rounded forms that one often sees in malachite are the result of how the mineral grows.  Ground water, interacting with rich copper deposits, typically produces a zone of complex minerals surrounding the copper-rich ore zone.  Malachite is one of these minerals.  It is a copper carbonate Cu₂CO₃(OH)₂.  Like the more familiar carbonate mineral that we know as limestone, which forms most instances of stalactites and stalagmites in caves, malachite frequently forms by precipitation from groundwater in open voids in the ground.  The resulting crystals can be botryoidal to stalagmitic masses, which, when cut, show concentric rings, much like one would see if they cut through a slow-growing stalagmite in a typical cave. (...Though cutting up stalagmites in caves is something you should never do.)  The bands of color represent minor differences in chemistry of the groundwater as the crystalline mass of malachite grew outward.  In case you would like to learn more, here is a link to the mindat page for malachite: http://www.mindat.org/min-2550.html  And here is a link to malachite on Wikipedia: http://en.wikipedia.org/wiki/Malachite