A star sapphire is a type of sapphire that exhibits a phenomenon known as asterism. This is a pattern of intersecting mineral inclusions that create a six-legged starlike pattern. The mineral is called rutile, which primarily consists of titanium dioxide.
Strength of Star
The main consideration for the value of a black star sapphire is how strong the star is. The star pattern can be very thin and hard to see in lower valued stones. The best value comes from a star that is clearly defined but does not have so much of the mineral inclusion to affect the clarity of the rest of the stone, according to the website "Ruby-Sapphire." The cost of star sapphires can range from $200 to millions, depending on the carat weight, the cut and the overall quality.
Color of Base
The base color of the black star sapphire is the next price consideration. A black sapphire is called a fancy sapphire, a designation that covers all sapphires that are not blue. The black color is can be caused by vanadium, iron impurities, or titanium inclusions. Many sapphires are heat-treated to increase the depth and hue of the color also. Star sapphires typically do not go under heat treatment as it can destroy the rutile inclusions. Clarity is a part of the color consideration as well. Some black star sapphires have too many inclusions to see through the stone clearly, which can greatly decrease the value. Pricing for an average star sapphire of 5-carat weight is approximately $1,000.
The best star sapphires, according to Ruby and Sapphire, are found in Mogok, Burma. This region has a depth of color that does not require heat treatment. Sri Lanka is another area where star sapphires can be found. Sri Lanka also produces the largest star sapphires, with specimens over 100 carats not unheard of.
Good quality star sapphires are inherently among the rarest of sapphires. The finest black star sapphires come from Chanthaburi, Thailand. The Thailand combination provides perfectly black, clear sapphires with golden star inclusions.
Black Star of Queensland
The Black Star of Queensland is the most famous black star sapphire. It is a 733-carat stone found in Queensland, Australia. It was discovered in 1933 and remains the largest black star sapphire found. It was cut as a cabochon, and has an unusual star pattern compared to other star sapphires. Instead of jutting out from the center of the stone, the star is located at the bottom. It is valued at over $50 million.