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Exhibit Tourmaline pegmatite The large groups of pink crystals of the boron-bearing mineral tourmaline and the tiny purple crystals of the lithium-bearing mineral... Exhibit Coal This coal contains metamorphosed organic matter (carbon) and brassy-colored iron sulfide minerals formed from decaying plants. Exhibit Coquina This calcium carbonate limestone is comprised of a group of fossil shells cemented by bituminous (organic carbon-rich) calcite. Exhibit Chalk This sample is from the chalk beds of southern England. Exhibit Limestone (Fossiliferous) This sample of limestone is composed of carbonate minerals, and formed in a shallow ocean. Exhibit Marble Deep burial of calcium carbonate-bearing rocks results in metamorphism, or transformation, of limestone to marble. Exhibit Claystone Weathering and erosion of rocks like granites concentrate elements that are necessary to form clay minerals, which accumulate as... Exhibit Gneiss With burial and the associated increase in pressure and heating, shales may be transformed by re-crystallization in the deep crust... Exhibit Chromium Ore Layers of chromite (black) are found near the bottoms of some magma chambers and are a source for the element chromium. Exhibit Ultra-high pressure rock The presence of diamond in this rock indicates that it had been buried to a depth of about 130 kilometers. Exhibit Coal Coal is the most abundant fossil fuel. It forms when dead and decaying plants are buried in swamps and marshes. Exhibit Red quartzite The rocks in the upper part of the Huronian Supergroup are red-brown, like this quartzite specimen.