Final answer:
Gem-quality corundum is rare due to the specific conditions required for the inclusion of chromium impurities that color the gems, coupled with high market demand and limited supply. Rubies, which are red corundum, owe their color to the presence of chromium occupying places in the crystal lattice normally filled by aluminum.
Step-by-step explanation:
Gem-quality corundum is considered rare because it requires specific conditions to form. Corundum in its purest form is colorless, but when trace amounts of chromium (Cr³⁺ ions) replace some of the aluminum (Al³⁺ ions) in its crystal structure, the gem takes on a vibrant color. Only about 1% of chromium content is enough to create a deep red ruby. The rarity is due to the specific environmental conditions needed for these trace elements to be present and to be incorporated into the corundum lattice. These conditions aren't commonly found, which is why high-quality rubies and other colored corundums are rare.
Another factor in the perception of rarity is the market demand versus supply. As consumers highly value these vibrant and durable gems, the limited supply that can meet the demand influences their rarity.
In the case of a ruby, the red color is specifically due to chromium impurities present in octahedral sites of the metal oxide lattice. Ruby is considered an allochromatic mineral, which means its color arises from these trace impurities rather than from the essential components of the mineral itself.