Sapphire, with its hardness of 9 on Mohs hardness
, is second only to diamond
, which renders it one of the most valuable and wearable gemstones of all-time. Sapphire is a 'precious four' gem and offers superb brilliance and rich, vibrant colors. Although most sapphire is heated or enhanced in other ways to improve color, clarity and stability, unheated and completely untreated natural sapphire can still be found. Usually unheated sapphire gems are found only in small sizes, but occasionally, large, clean stones are uncovered too. Because unheated sapphire is such a rarity, they are indeed relatively expensive, particularly unheated top-color blue 'Ceylon' sapphire. Unheated, natural sapphire is mostly sourced from Africa, though Australia and parts of Asia are known to produce some fine materials too.
As of recent times, Madagascar and Tanzania are two of the most important sources for fine untreated, unheated sapphire, as is Thailand. Tanzanian sapphire is usually pink to violet in color (often violet-blue), and many of the specimens exhibit very noticeable color change properties. Color-change unheated sapphire can be very expensive and is highly sought-after by both jewelers and gem collectors. Fine quality unheated sapphire gems from Madagascar, as well as Australia, display silk inclusions, which are highly desirable. The silky appearance is cause by rutile inclusions and the presence of such silk is usually a good indicator that the gem has not been heated, since rutile inclusions melt at a much lower temperatures than those at which sapphire is heated. Sapphire is also the official birthstone for those born in the month of September.