It is the next-hardest stone on earth after the diamond, and the most popular of the semi-precious stones. Corundum is the name of the mineral, it’s an aluminum oxide. Many high-end watches are made with corundum (probably lab-grown) for the face crystal because it is so durable and scratch-resistant. Corundum minerals originated deep inside the earth 500 million to 2.5 billion years ago in the magma with high temerature and pressure. Long periods (perhaps even billions of years) of erosion carried these stones to the foothills, flood plain of rivers, and streams where they are mined.
Three varieties of gemstones come from this mineral:
Sapphire, most popular in its blue form, but appearing in all the colors
(except red - that’s a ruby), as well as clear, black, gray, and brown;
color-change sapphires from Tanzania exhibit different colors
in different light;
Ruby - appearing only in red;
Padparadscha (comes from Sanskrit: padma = lotus raga = color)
a pinkish-orange (like a lotus) that is very rare in its natural, untreated form.
Originally it was from Sri Lanka, but is also found in Vietnam and Africa.
Sapphires are found in Australia, Thailand, Sri Lanka, India, China (Shandong), Madagascar, East Africa, and North America (mostly Montana).
Star Sapphire
This sapphire exhibits a star-like phenomenon called asterism. It is a needle-like inclusion of a mineral called rutile, which is titanium oxide. It creates a 6-rayed “star” pattern. The stone is cut en cabochon with the center of the star at the top of the dome. The value of a star sapphire does not depend solely on weight, but also on the visibility of the asterism and body color.
http://atlantadiamond.com/2120/sapphire-septembers-birthstone/
