Sapphire and ruby are varieties of natural:

aluminium oxide

The correct answer is option 1. aluminium oxide.

Sapphire and ruby are both varieties of the mineral corundum, which is composed of aluminum oxide \((Al_2O_3)\). The difference in color between sapphire and ruby is due to trace amounts of impurities. Sapphire typically comes in various colors, including blue, yellow, and pink, while ruby is the red variety of corundum. The red color in ruby is caused by the presence of chromium impurities in the crystal structure of corundum.