Star Ruby and Star Sapphire: The Complete Guide

A star ruby or star sapphire moving under a single light source is one of the most hypnotic sights in the gem world. The six-rayed star — bright, sharp lines radiating from the centre of the dome — glides across the surface as the stone turns, seemingly alive in a way that no faceted gem replicates. Fine star rubies and star sapphires from Burma and Sri Lanka have been prized by royalty, collected by museums, and admired by gem lovers for thousands of years. The Star of India sapphire, the DeLong Star Ruby, the Rosser Reeves Star Ruby — these are among the most celebrated objects in the natural world.

The Science of Asterism

The star effect (asterism) in corundum is caused by needle-like inclusions of rutile (titanium dioxide) that grew within the sapphire or ruby crystal parallel to specific crystallographic planes during the stone’s formation. In the trigonal crystal system of corundum, rutile needles align along three sets of planes spaced 60 degrees apart. When the stone is cut as a cabochon with the dome perpendicular to the c-axis, light reflecting from these three sets of needles creates a six-rayed star. The star appears at the point on the dome where reflected rays from all three sets of needles converge — directly above the stone when it is properly oriented.

The rutile that creates the star is the same rutile that, when dissolved by heat treatment, improves the clarity and colour of corundum. This is the central trade-off of star corundum: heat treatment to improve colour eliminates the rutile silk and therefore the star. Untreated star stones preserve their silk — and therefore their star — but may have less optimal colour than heat-treated non-star material. The finest star stones combine good body colour, a sharp and well-centred star, and no heat treatment.

Star Ruby Quality Factors

A fine star ruby is assessed on: body colour (the ideal is a rich, saturated red — pigeon’s blood quality in a star is extraordinarily rare and valuable), star quality (sharpness, straightness, centring, and visibility in low light), and overall transparency and lustre of the cabochon. The star should be visible not just under direct light but should remain at least partially visible in lower light conditions.

The finest star rubies come from Burma (Mogok), where marble-hosted, high-chromium, low-iron corundum produces the richest red body colour. Star rubies from Thailand, Madagascar, and Sri Lanka also appear commercially. Very fine Burmese star rubies of 5-10 carats with rich colour and a sharp, well-centred star can command prices of tens of thousands of dollars per carat. The DeLong Star Ruby (100.32 carats) in the American Museum of Natural History is one of the finest large star rubies known.

Star Sapphire Quality Factors

Star sapphires are more abundant than star rubies and cover a wider range of body colours. Blue star sapphires are the most commercially significant; grey star sapphires (often with fine stars and high transparency) are prized by collectors; black star sapphires from Thailand and Australia show stars against a dark body. The Star of India (563.35 carats) at the American Museum of Natural History and the Star of Lanka at the Smithsonian are among the most famous gem specimens in existence.

Quality criteria mirror those for star ruby: body colour quality, star sharpness and centring, and cabochon quality. Fine blue star sapphires from Sri Lanka with vivid blue body colour and a strong, well-defined star are the benchmark. Some Sri Lankan star sapphires show both the star and transparency simultaneously — allowing light to pass through the stone while the star is still visible — creating an effect called “transparent star” that is exceptionally rare and desirable.