Sapphires are composed of a material known as corundum, which in its purest form is colorless. A nine out of ten on the Mohs Scale of Hardness, corundum is the second hardest natural substance after diamonds. While sapphires can come in a variety of different colors, depending the mineral content of the stone, deep blue is the most famous color and therefore the most symbolically charged.

Since blue is the color of the sky, sapphires are strongly associated with the heavens, and some cultures believe it to be a divine stone. In Christianity, sapphires are emblematic of wisdom, purity and chastity. In India and parts of Southeast Asia, the sapphire is one of the navaratnas, a series of nine specific gems that are used to represent the heavenly bodies. In this system, the blue sapphire represents the planet Saturn, while the yellow sapphire represents Jupiter.

In some individual sapphires, there can occur a series of three pale inclusions that cross each other at the center to form a six-pointed star. Due to their high value and the belief that they can bring good luck, these sapphires are considered particularly auspicious. One interpretation of the three lines is that they represent destiny, faith and hope.

Sapphires are also the traditional birthstone for the month of September.