Final answer:
Gem corundum, which includes gems like rubies, can develop in three geological settings: igneous, where they form from cooling magma; metamorphic, where they emerge from pre-existing rocks altered by heat and pressure; and sedimentary, where they are transported and deposited by water or other forces after forming in the other two settings.
Step-by-step explanation:
The geological settings/genetic models for gem corundum, such as rubies and sapphires, consist of three main types. The first type is igneous-origin corundum, where crystallization occurs as magma cools either deep within the Earth or after erupting onto the surface. The second type includes metamorphic-origin corundum, formed by the transformation of pre-existing rocks through heat and pressure, leading to the recrystallization and formation of corundum.
Finally, there are corundum deposits associated with sedimentary processes which occur when corundum crystals originating from igneous or metamorphic rocks are transported and deposited in a new environment, such as river beds. Gem-quality corundum like rubies owes its color to the presence of Cr3+ impurities within its structure which replace some of the Al3+ ions, providing the deep red hue of the ruby. Traces of these impurities create the vibrant colors seen in gem-quality corundum.