Marble with ruby;
Karakoram Mountains, Pakistan
Pure limestones, made of calcium carbonate, are converted
into crystalline calcite marble (also calcium carbonate)
during metamorphism. However, if the limestone contains
impurities, other minerals may form in the marble. This rock
had aluminium as its principal impurity (probably in the
form of clay particles), and high grade metamorphism has
produced large crystals of corundum (aluminium oxide). There
must also have been tiny amounts of the metal chromium,
which colours the corundum red - this is the gemstone ruby.
The coarse interlocking texture of the calcite crystals can
also be made out.