Star Sapphires and Star Rubies: How the Star Forms and How to Choose One

A star sapphire or star ruby looks like an ordinary smooth cabochon until light hits it. Then a crisp six-rayed star appears on the dome and glides across the stone as you move it. Here is how the star forms, what separates a fine star from an average one, and how to buy one with confidence. Every stone in the photos and videos below is a natural star sapphire or star ruby from our own stock, and all of them are for sale.

Star sapphires in blue and violet-blue, a purplish-red star ruby (far left) and cat's-eye cabochons, under a single light. All natural and for sale.

What makes the star? Asterism explained

The star is called asterism. Sapphire and ruby are both corundum, and some crystals grow with millions of tiny needle-like inclusions, usually rutile, which gemmologists call "silk". The needles line up in three directions at 60 degrees to each other, following the six-sided structure of the crystal. When light reflects off each set of needles it forms a bright band, and three bands crossing make a six-rayed star.

A few points follow from this:

• The stone must be cut as a cabochon. A domed, unfaceted top focuses the reflected light into sharp rays. A faceted cut would scatter it.
• The cutter has to orient the rough correctly. The base must sit at right angles to the crystal's main axis, or the star appears off-centre or with uneven legs.
• The star needs a single light source. Sunlight, a spotlight or a phone torch brings it out. Under soft, diffuse light it fades. That is why the star seems to move: it follows the light.
• The silk also affects the body. The same inclusions that create the star give the stone its soft, velvety glow, so star stones are translucent rather than glassy.

Some stones show a twelve-rayed star when two kinds of needle inclusions are present together. These are rare and highly collectable.

Watch the star move

Photos only hint at asterism. These short clips show what you see in hand: the rays sharpen, glide and recentre as the light moves.

Left: star sapphires on a display tray. Centre and right: a box of mixed star sapphires and star rubies. Ask us for a video of any single stone.

Star sapphire colours

Star sapphires come in more colours than most people expect:

• Blue and grey-blue. The classic star sapphire. The most valued stones pair a saturated cornflower to royal blue body with a sharp silver-white star.
• Violet and purple. Cooler and moodier, often with a very bright star against the darker body.
• Yellow and golden. Warm, honey-toned stones where the star reads almost white-gold.
• Pink. Soft pink star sapphires sit just below star ruby in colour saturation.
• Black star sapphire. An opaque, dark body with a golden or silvery star, often from Thailand.

Left: blue, violet and yellow star sapphires. Right: a 91-piece parcel of star sapphires and star rubies, 207.67 ct in total. Stones can be bought individually, or the parcel whole for jewellers.

Star rubies

A star ruby is simply red corundum with asterism. The best show a strong red to purplish-red body with a star that stays sharp right to the edge. Because the silk that creates the star also softens the colour, a vivid red star ruby with a crisp star is much rarer than either feature alone. Star rubies are the July birthstone in their own right, as star sapphires are for September.

Two famous examples show how prized they are: the 138.72 ct Rosser Reeves Star Ruby at the Smithsonian, and the 563 ct Star of India sapphire at the American Museum of Natural History in New York.

Left: a set of twelve star rubies, ideal for a matched pair or a single statement ring. Right: the 207.67 ct parcel in motion. All natural and for sale.

How to judge a star sapphire or star ruby

1. Star sharpness. The rays should be fine, bright and continuous, not blurry or broken.
2. Straight, equal legs. All six rays should be straight, roughly equal in length and ideally reach the edge of the stone.
3. Centring. When the stone faces up under a light directly overhead, the star should sit at the top of the dome.
4. Mobility. A good star glides smoothly across the dome as you tilt the stone or move the light.
5. Body colour. Richer colour adds value, but the balance between colour and star matters most. A dark stone with a faint star is worth less than a lighter stone with a brilliant one.
6. Translucency and clarity. More translucent stones glow. Surface-reaching fractures and cavities in the dome lower value.
7. Proportions. Cabochons are often cut with a heavy base to save weight. A sensible dome and base sit better in a setting and are fairer value per carat.

Natural, synthetic and treated stars

Lab-grown star sapphires and rubies have been made since the 1940s and are inexpensive. Their stars tend to look too perfect: very sharp and uniform rays on a perfectly even, often opaque body, sometimes with curved growth lines under magnification. Some natural stones also have a star created artificially by diffusing chemicals into the surface. In those stones the star sits only on the skin of the stone.

Natural stars usually show slight irregularities: small variations in the rays, colour zoning and visible silk. Because strong heat can dissolve the silk that forms the star, star stones are commonly sold unheated. Treatment still needs to be confirmed stone by stone. We disclose what we know about every stone, and for important purchases a lab report from a gemmological laboratory gives independent confirmation. See our guide on how to tell if a sapphire is real.

Setting and wearing a star stone

• Bezel and low prong settings protect the dome and suit the soft, rounded look of a cabochon.
• Leave the top open to light. The star needs light from above, so avoid deep or heavily covered settings.
• Durable for daily wear. Corundum is 9 on the Mohs scale, second only to diamond, so star sapphires and rubies suit rings worn every day.
• Easy care. Clean with warm water, mild soap and a soft brush.

FAQ

Are star sapphires real sapphires?

Yes. A natural star sapphire is corundum, the same mineral as a faceted sapphire, with silk inclusions that create the star. Lab-grown star sapphires also exist and should always be disclosed.

Why can't I see the star indoors?

Asterism needs a single, direct light source. Under ceiling lights or diffuse daylight the star softens. Try sunlight or a phone torch held above the stone.

Is a star ruby a ruby or a sapphire?

Both are corundum. Red stones are called ruby and all other colours are called sapphire, so a red star stone is a star ruby and a pink, blue, violet or yellow one is a star sapphire.

Can I buy the stones in this article?

Yes. Every stone shown is natural and for sale from Roho Jewels. Message us and we will send details and a video of the stones that match what you are looking for.