Star Sapphire and Star Ruby: Asterism Explained

Before the trade learned to prize transparency above everything, it prized the star. A star sapphire or star ruby is corundum that grew with so much rutile silk, in such orderly directions, that a cabochon cut from it throws a six-rayed star across its dome that moves as the stone moves. The Star of India, the Rosser Reeves ruby, the great stones in the old crowns are stars, and they were bought for the phenomenon at a time when a clean transparent sapphire was a lesser thing. The market has inverted since. The theory has not.

What it is

Ordinary corundum, Al₂O₃, hardness 9, specific gravity near 4.00, in ruby or in any sapphire colour, with one addition: silk. As corundum cools, titanium that was dissolved in the lattice comes out of solution as needles of rutile, and the crystal structure forces those needles to grow in three directions in the basal plane, crossing at sixty degrees. Light striking each set of parallel needles reflects as a band at right angles to them, and three bands crossing make a six-rayed star. The star lives in the stone, not on it; it is visible only when the cabochon is cut with its base perpendicular to the crystal’s vertical axis, and it sharpens as the dome rises. Cut the same rough at the wrong angle and the star slides off the side and disappears. In rare stones a second mineral, hematite or ilmenite, grew its own set of needles at a different angle, and the stone throws two stars, twelve rays, which the black stars of Thailand and Cambodia show most often.

Where the colour comes from

The same guests as in transparent corundum. Chromium gives star ruby its red and the pink of most so-called star rubies, which are pink sapphires with a star and should be priced as such. Iron and titanium give the blue of star sapphire, and here the trade meets a contradiction: the titanium that makes the silk is the titanium that would have made a deeper blue had it stayed in solution, so the finest star sapphires tend toward a lighter, greyer blue than the finest transparent ones, and a strongly coloured stone with a sharp star is rare on two counts. Silk also scatters light, softening and spreading the colour across the dome, which is the same effect that makes Kashmir sapphire velvety when the silk is fine enough not to form a star.

Where it comes from

Burma produced the great star rubies, from Mogok, in a red that transparent stones from the same ground made famous. Sri Lanka is the historic source of fine star sapphire and of the largest examples: the Star of India, 563 carats, is Sri Lankan, as is the Star of Adam, over fourteen hundred carats, found in 2015. Thailand and Cambodia gave the iron-rich black and dark blue stars, often with twelve rays. Tanzania, Madagascar, Vietnam and India supply the modern commercial market, much of it in the greyish and purplish tones that heavy silk brings. Origin is an opinion for stars as for any corundum, and a Burmese call on a star ruby moves the price as it does on a transparent one.

How it is judged

Body colour first, because a star never makes a pink stone red or a grey stone blue, and the price is set by colour before the phenomenon is considered. Then the star: sharp rather than fuzzy, centred on the dome, complete in all six rays to the edge, and mobile, gliding across the stone as it turns rather than sitting fixed. Then the balance between transparency and silk, which is the star’s essential compromise: enough needles to throw a strong star, few enough that the stone keeps colour and some depth rather than going milky. Then the cut, which should be a well-proportioned dome with the star centred and the base not so deep that weight is hidden below the girdle, a habit of native cutting as old as the stones. Then size, where a star ruby of true red with a sharp star above a few carats is one of the rarest objects in corundum, and star sapphires reach the great sizes that transparent stones never do.

Treatments, and what to demand on paper

The silk that makes the star is the silk that heat dissolves, so most star corundum is unheated, and the treatment line on its report is usually the cleanest in the trade. That is the good news. The rest is identity. Synthetic star corundum has been grown since 1947, and the early stones were so perfect, with rays too sharp and too regular and a body colour too even, that the perfection itself gives them away; later ones are better, and a report settles it. Diffusion-induced stars are created by driving titanium into the surface of an ordinary stone and heating it so silk forms in a thin skin, giving a star that vanishes if the stone is repolished; laboratories detect this and state it. Glass filling appears on poor material. The unheated line, for once, is rarely the problem; the identity and treatment lines above it are.

The market

A smaller market than transparent corundum, and a more patient one. Fine star sapphires with good colour and sharp stars trade well below transparent stones of comparable colour, which for a buyer who wants the phenomenon is an opportunity. Star rubies of Burmese red are the exception: the Rosser Reeves star ruby in the Smithsonian, at 138 carats, and a few others define the top, and a serious Burmese star ruby of size with paper is priced like the rarity it is. Black star sapphires are an ornamental market. Between the two sit the grey-blue Sri Lankan and African stones that most jewellers know, priced by star quality and colour, with size rewarded less steeply than in any other corundum.

Care

Hardness 9 and the cabochon cut make star corundum nearly indestructible in wear; there are no facet edges to chip and no cleavage to split. Ultrasonic and steam are safe for untreated stones. A diffused star must never be repolished. Beyond that the stone asks for nothing but a single light source, because a star needs one point of light to form, and under diffuse office lighting it goes to sleep.

Where I stand

I do not hunt stars often, because most of my clients want transparency, but I keep an eye open for two things: a Burmese star ruby of true red, which is rarer than most transparent rubies and priced by people who understand that, and a Sri Lankan star sapphire of strong blue with a sharp star, which is undervalued against its transparent cousins by a market that forgot what it once paid for. The examination is the same as for any corundum, with one light instead of two: colour in daylight, then the star under a single lamp, then the paper.

Questions collectors ask

What causes the star in a star sapphire?

Silk: fine needles of rutile that crystallised inside the corundum in three directions crossing at sixty degrees. Light reflecting off each set of needles forms a band, and the three bands meet as a six-rayed star on a cabochon cut with its base perpendicular to the crystal's main axis. A second set of needles of another mineral gives the rare twelve-rayed star.

Are star sapphires and star rubies heated?

Usually not, because the heat that improves colour dissolves the silk that makes the star. That makes star corundum some of the most honestly untreated material in the trade, with two exceptions to check for: synthetic stars, made since 1947 with unnaturally perfect rays, and diffusion-induced stars created on the surface of ordinary stones.

What makes a star ruby valuable?

The body colour first, because a star does not make a pink stone red; then the star, which should be sharp, centred, complete in all six rays and mobile as the stone turns; then the balance between transparency and the silk needed for the star; then size. A Burmese star ruby of true red with a sharp star above a few carats is among the rarest objects in corundum.