Final answer:
Corundum is a mineral composed of aluminum oxide, while ruby and sapphire are gem-quality varieties of corundum. The color of ruby is due to the presence of chromium impurities.
Step-by-step explanation:
The basic relationship between corundum and ruby+sapphire is that corundum is a mineral composed of aluminum oxide (Al2O3), while ruby and sapphire are gem-quality varieties of corundum. The color of ruby is due to the presence of a small amount of chromium (Cr3+), which replaces some of the aluminum ions in the crystal structure of corundum. Other impurities produce a wide variety of colors possible for sapphires.