Rare Gems

Of all the optical phenomena that gemstones can display, none is more dramatic or more immediately captivating than asterism — the six-rayed star that appears to float across the surface of a star sapphire when light falls upon it. The star moves as the stone rotates, always centred under the light source, always perfectly formed in fine specimens. It seems almost supernatural, and for centuries people believed these gems were inhabited by a living spirit.

Sri Lanka produces star sapphires of extraordinary quality — both blue and black varieties, and more rarely pink and grey. The famous Logan Blue Star Sapphire (422.99 carats), one of the largest and finest star sapphires in the world, is a Ceylon stone now housed in the Smithsonian Institution's National Museum of Natural History. It is one of the finest examples of what Sri Lanka's gem deposits can produce.

What Creates the Star?

The star in a star sapphire is created by a phenomenon called asterism — from the Greek word for star. It is caused by the presence of tiny needle-like inclusions of rutile (titanium dioxide) oriented in three sets of parallel directions within the corundum crystal. Each set of needles is oriented 60 degrees apart, following the crystal's trigonal symmetry. When light strikes the cabochon surface, each set of needles reflects a band of light. Three bands intersecting at 60 degrees create a six-pointed star.

The quality of the star depends on the density, regularity, and orientation of these rutile inclusions. Too few needles and the star is faint. Too many and the stone becomes opaque and dark. The finest star sapphires have just the right density of silk — enough for a sharp, bright star against a translucent to semi-transparent body colour.

This is why heat treatment, which is routine for transparent blue sapphires, is counterproductive for star sapphires. Heat treatment at the temperatures used for colour enhancement dissolves the rutile silk that creates the star. All star sapphires on the market should be unheated — though this does not mean they command a premium on par with unheated transparent sapphires, since the market for stars is different.

How to Evaluate a Star Sapphire

Star quality is the most important factor. The ideal star has six sharp, straight, equally weighted rays that meet precisely at the centre of the stone. The star should be clearly visible under a single directional light source — a pen torch or a bright point of light works best — and it should move smoothly as the stone is rotated. Faint, wavy, or asymmetric rays indicate lower quality silk formation.

Body colour matters significantly for value. For blue star sapphires, the body colour should be a pleasing medium blue — too pale and the contrast between star and body is weak; too dark and the stone appears black and the star is hard to see. The finest Ceylon blue star sapphires have a rich, velvety blue body colour that perfectly showcases the star.

Transparency separates fine star sapphires from common material. The finest specimens are translucent — light passes through them partially, giving the stone depth and life. Opaque star sapphires appear flat and dull regardless of star quality. Semi-transparency with a good star is the ideal combination.

Cut is critical. Star sapphires are always cut as cabochons — polished domes without facets. The dome height must be precisely calibrated to centre the star at the apex of the dome. A dome that is too flat pushes the star off-centre; too high and the stone appears dark. A master cutter in Ratnapura will spend considerable time orienting the rough before cutting to ensure the star is perfectly centred.

Ceylon vs Other Origins

Star sapphires are also found in Myanmar (Burma), Thailand, and Australia. Burmese star sapphires tend to be darker and less transparent than Ceylon material. Thai stars are often greyish or have a weaker star. Australian star sapphires are typically very dark blue to black. Ceylon star sapphires are generally considered the finest in terms of the combination of body colour, transparency, and star quality — the three factors that together create a truly exceptional stone.

Black star sapphires are also found in Sri Lanka — these show a white or silver star against a black or very dark blue body. They are significantly less expensive than blue star sapphires but can be very striking, particularly in large sizes.

Price Guide

• Commercial star sapphire (weak star, opaque): USD 20–100 per carat
• Good quality (sharp star, semi-translucent, fine blue): USD 200–800 per carat
• Fine quality (sharp centred star, translucent, rich blue): USD 800–3,000 per carat
• Collector quality (exceptional star, fine colour, large size): USD 3,000–15,000+ per carat

Star sapphires are generally not certified by major laboratories for origin and treatment in the same way transparent sapphires are — purchases are largely based on visual assessment. For stones above USD 2,000, a NGJA or basic GIA identification report is still recommended to confirm the stone is natural corundum rather than synthetic or assembled.

Disclaimer: Price ranges are approximate market references. CeylonGemLuxury does not sell gemstones directly.