Ruby and Sapphire Origins and Geology
The mineral corundum forms in metamorphosed crystalline limestones and dolomites, as well as other metamorphic rock types such as gneiss and schist. It also occurs in igneous rocks, such as granite and nepheline syenite. Miners often find gem corundums in placer deposits.
Ruby historically comes from the Mogok stone tract. The history of the mines here is long, complex, and turbulent. Gems occur in a gravel layer called byon at a depth of 20 to 100 feet. Miners recover them by washing and screening with broad screens, then handpicking promising pebbles. Corundum originates in metamorphic marbles that have largely weathered away. This is the source of the world’s finest rubies.
Fine sapphires occur in northern India in the NW Himalayas at an elevation of nearly 15,000 feet, in a deposit snowed under most of the year. Gems occur in a pegmatite and in the valley below, in…