Ruby is red corundum — aluminium oxide coloured by chromium — and in the Navratna tradition it is the stone of the Sun (Surya). It is also the most valuable coloured gemstone per carat at the fine end, which makes it the most imitated, most synthesised and most heavily treated stone in the trade. A "ruby" bought without testing may be any of a dozen things.
What ruby is
Corundum with enough chromium to be red. Pink corundum is pink sapphire; the boundary is a judgement that varies between laboratories and markets. Properties: refractive index 1.762–1.770 (two readings), specific gravity 4.00, hardness 9, doubly refractive, pleochroic (purplish red / orangey red), chromium absorption spectrum, usually red fluorescence under long-wave UV.
What is sold as ruby
Red garnet (almandine, pyrope, rhodolite). The commonest natural substitute. Singly refractive, no pleochroism, RI 1.74–1.83 (one reading), SG 3.7–4.3, iron spectrum with three bands in the yellow-green. Separated in seconds on the polariscope and dichroscope.
Red spinel. Historically confused with ruby (the "Black Prince's Ruby" is spinel). Singly refractive, RI 1.718, SG 3.60, chromium spectrum, red fluorescence. A valuable stone in its own right; a problem only when sold as ruby.
Red glass. Singly refractive, RI variable, gas bubbles and swirls under magnification, often with a cobalt-free but obviously non-chromium spectrum. Trivial to detect.
Synthetic ruby. The same material as natural — identical RI, SG, spectrum, fluorescence. Separated by inclusions: curved striations and gas bubbles (flame fusion); flux veils and platinum platelets (flux-grown). How synthetic gems are grown.
Glass-filled ruby. Natural corundum, heavily fractured, impregnated with lead glass. Bubbles in fractures, blue flash effect, crazed texture. Sold in large quantities at prices that seem too good. Glass-filled ruby: the composite stone explained.
Composite and assembled stones. Garnet-topped doublets (a slice of garnet fused to glass) and other assembled stones, detectable at the join under magnification and by immersion.
Treatments in genuine ruby
Heating. Very common, to improve colour and clarity. Permanent and stable. Detected by altered silk, tension haloes around crystals, and other microscopic evidence; low-temperature heating can be undetectable without spectroscopy. Heat treatment in ruby and sapphire.
Heating with flux residues. Heating in the presence of borax or similar flux that heals fractures and leaves glassy residues. Reported as such.
Glass filling. Extensive filling, as above; graded by extent.
Surface diffusion and coating. Uncommon in ruby but possible.
How the laboratory examines a claimed ruby
Refractometer (1.762–1.770, two readings) and polariscope (blinking) — confirms corundum, eliminates garnet, spinel and glass. Dichroscope confirms pleochroism. Spectroscope confirms chromium. UV notes fluorescence. Microscope decides natural versus synthetic, and looks for heating evidence, flux residues and glass filling. Report states species, natural or synthetic, and treatment with extent. How a gemmological laboratory tests a gemstone.
Reading a ruby report
A complete finding reads something like: Natural corundum (ruby), indications of heating, no filling observed, or Natural corundum (ruby), glass-filled — composite, or Synthetic corundum (ruby), flame fusion. Each is a different article at a different price. A report that says only Ruby — Natural has left the most important questions unanswered.
Frequently asked questions
How can I tell ruby from garnet at home?
With a dichroscope or two polarising filters: ruby shows two colours and blinks; garnet shows one colour and stays dark. Without instruments, you cannot reliably.
Is a heated ruby still natural?
Yes. Heating alters a natural crystal. The report should say both natural and heated.
What is a "Burma ruby"?
A ruby whose origin is stated as Myanmar (Burma). Origin is an opinion based on inclusions and chemistry, commanding a premium; see Can a laboratory tell where a gemstone came from?.
Are all inexpensive rubies glass-filled?
Not all, but a transparent red corundum of good size at a low price is very often glass-filled or synthetic. The price is the first clue; the microscope is the answer.
Does ruby have to be a particular weight for astrological use?
That is a matter for the wearer's tradition. The laboratory reports weight in carats and grams so that any requirement can be checked. Ratti, carat and gram.