Blue sapphire is corundum coloured by iron and titanium, and in the Navratna it is the stone of Saturn (Shani). Tradition treats it with particular caution — it is often "tested" by the wearer for a trial period — and the market treats it with particular creativity. Synthetic sapphire has been manufactured for over a century; blue glass and synthetic spinel imitate it cheaply; and nearly all natural blue sapphire has been heated.
What blue sapphire is
Corundum in any blue shade, from pale to nearly black. Properties: RI 1.762–1.770 (two readings), SG 4.00, hardness 9, doubly refractive, pleochroic (violet-blue / greenish blue), iron absorption bands in the blue, usually inert under UV (some heated stones show a chalky blue-white glow under short-wave).
What is sold as blue sapphire
Synthetic blue spinel. The classic cheap imitation. Singly refractive (dark on the polariscope), RI 1.728 (one reading), SG 3.64, red fluorescence under long-wave UV that no blue sapphire shows, and a cobalt spectrum. Caught in seconds.
Blue glass. Singly refractive, RI variable, bubbles and swirls, cobalt spectrum in most cases.
Synthetic sapphire. The same material; identical RI, SG and pleochroism. Separated by curved striations and gas bubbles (flame fusion) or flux and platinum (flux-grown). Flame-fusion synthetic sapphire is extremely common and often shows Plato lines (fine intersecting lines seen in immersion between crossed polarisers) as a further indicator.
Tanzanite. A natural blue-violet zoisite, valuable itself. RI 1.69–1.70, SG 3.35, strongly trichroic (blue / purple / brownish). Sold as sapphire mainly by mislabelling.
Iolite. Natural blue cordierite, inexpensive. RI 1.54–1.55, SG 2.6, extreme trichroism (violet-blue / pale blue / yellowish).
Kyanite, blue tourmaline, blue topaz, blue zircon. Each with its own RI and SG, none matching corundum.
Doublets. A thin sapphire crown on a glass or synthetic base. Join visible at the girdle under magnification and in immersion.
Treatments in genuine sapphire
Heating. The norm. Improves colour and clarity, dissolves silk. Detected by altered silk, haloes around crystals and chalky UV fluorescence. Unheated sapphire of fine colour is rare and commands a premium; claims of "unheated" deserve a report that states what evidence supports the claim. Heat treatment in ruby and sapphire.
Surface (titanium) diffusion. Blue cooked into a thin skin of pale corundum. Colour concentrated at facet junctions in immersion; repolishing removes it. Diffusion-treated sapphire.
Beryllium diffusion. Used mainly for yellow and orange but can enhance blue. Needs advanced instruments to exclude.
Cobalt-glass filling and cobalt diffusion. Newer treatments that add blue via glass in fractures or cobalt at the surface, producing a cobalt spectrum in what tests as corundum — a combination that should not occur naturally and flags the treatment.
How the laboratory examines a claimed blue sapphire
Polariscope (blinking) and dichroscope (two blues) eliminate spinel, glass and every singly refractive imitation immediately. Refractometer confirms corundum. Spectroscope checks for the iron spectrum and against cobalt. UV checks for the red glow of synthetic spinel and the chalky glow of heating. Microscope separates natural from synthetic and looks for heating and diffusion evidence; immersion checks colour distribution. Report states species, origin type, and treatment.
Reading a sapphire report
Natural corundum (blue sapphire), indications of heating is the most common honest finding. No indications of heating is a premium finding and should be backed by observed silk or equivalent evidence. Synthetic corundum or colour induced by diffusion changes the article entirely.
Frequently asked questions
Does a real sapphire glow under UV?
Most blue sapphire is inert. A blue stone that glows red is synthetic spinel. A chalky blue-white glow under short-wave suggests heating.
Is heated blue sapphire acceptable?
It is natural sapphire, permanently improved, and the great majority of the market. Whether it meets a particular tradition's requirements is for the wearer to decide with the fact disclosed.
What is "Ceylon" sapphire?
A trade name for Sri Lankan origin, often lighter and brighter in tone. Origin is an opinion; see Can a laboratory tell where a gemstone came from?.
Can a dark, almost black stone be sapphire?
Yes; very dark blue sapphire is common and inexpensive. Tone is a quality factor, not an identity one.
How is neelam weighed for astrological purposes?
The laboratory reports carats and grams; conversion to ratti is straightforward. Ratti, carat and gram.