The Beauty Behind the Star – What Makes a Star Sapphire a Star?

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Not every gemstone is meant to sparkle with facets.

Some of the most captivating stones in the world reveal their beauty through a much older and seemingly simpler style of cutting: the cabochon.

Smooth, polished and softly domed, a cabochon — often affectionately called a “cab” — has no facets. Instead of being cut to create flashes of reflected light, the stone is shaped and polished to showcase its colour, character and, in certain exceptional gems, a remarkable optical phenomenon hidden within.

What Is a Cabochon?

A cabochon is a gemstone that has been shaped and polished rather than faceted. It will typically have a smooth, rounded dome with a flat or slightly curved base.

The style is particularly suited to translucent and opaque gemstones, or stones whose inclusions create unusual optical effects.

While a faceted cut is designed to manipulate light through a series of precisely angled surfaces, the beauty of a cabochon comes from its uninterrupted surface. Colour appears softer and deeper, inclusions become part of the stone’s character, and certain natural phenomena can suddenly become visible.

Among the most extraordinary of these is asterism — the appearance of a star.

What Makes a Star Sapphire a Star?

A star sapphire does not have a star sitting on its surface. The phenomenon comes from within the gemstone itself.

Sapphires are composed of corundum, and some contain extremely fine, naturally oriented needle-like inclusions, commonly rutile — sometimes referred to as “silk.”

When light strikes these microscopic inclusions, it is reflected and scattered into narrow bands of light. Where those bands intersect, they create what appears to be a luminous star floating across the surface of the gemstone.

This phenomenon is known as asterism.

It is also why the cabochon cut is so important. The curved dome concentrates the reflected light into visible bands. A skilled cutter must study the rough gemstone carefully and orient the cabochon correctly so that the star appears as close as possible to the centre of the dome.

When the stone is moved beneath a single light source, the star appears to glide gracefully across its surface.

It is one of those wonderful occasions in gemology where an inclusion — something that might be considered an imperfection in another gemstone — becomes the very thing that makes the stone extraordinary.

Why Do Some Stars Have More “Legs” Than Others?

The “legs” of a star are more properly called rays, and the number of rays is determined by the internal structure of the gemstone and the directions in which its microscopic inclusions are oriented.

Star sapphires and star rubies most commonly display six rays. Three directions of aligned inclusions create three intersecting bands of reflected light, producing the familiar six-rayed star.

Other gemstones can produce different patterns. Some star gems display four rays, while particularly unusual sapphires can exhibit twelve-rayed stars.

A twelve-rayed sapphire can occur when two different sets of inclusions are present and oriented differently within the same stone. The result is essentially two six-rayed stars overlapping one another.

The quality of a star is not determined simply by the number of rays, however. A particularly beautiful star will be sharp and well defined, properly centred, and have rays that extend evenly across the cabochon. Most importantly, the star should move smoothly across the gemstone as the stone or light source moves.

The Importance of Light

Star sapphires are wonderfully responsive to their surroundings.

Their asterism is generally most dramatic beneath a strong, relatively concentrated light source. Natural daylight can be particularly beautiful, revealing subtleties of colour and character that artificial lighting may not reproduce in quite the same way.

This is why a star sapphire can look remarkably different as it moves from one lighting environment to another.

Turn the stone gently in the light and its star seems almost alive.

Natural Stars — And the Famous Linde Stars

Not every star sapphire is natural.

One particularly interesting chapter in twentieth-century gemology began with the Linde Air Products Company, which developed synthetic star ruby and star sapphire.

Linde's man-made star stones were publicly announced in September 1947, although the company stated that the method had been discovered during the early years of the Second World War and that wartime demands had delayed commercial production.

These stones became known as Linde Stars and are an interesting part of mid-century jewellery history.

They were genuine synthetic corundum — possessing essentially the same fundamental mineral composition as ruby and sapphire — but grown by man rather than formed naturally within the earth. Their asterism was deliberately produced through carefully controlled internal needle-like structures.

Today, an original Linde star sapphire can be an appealing period object in its own right, particularly when it survives in an original mid-century setting. It should, however, always be distinguished from a naturally formed star sapphire.

Beauty in Imperfection

Perhaps the most fascinating thing about a star gemstone is that its beauty depends upon what lies inside it.

The microscopic inclusions responsible for the star might ordinarily be described as imperfections. Yet without them, there would be no star at all.

A fine star sapphire is therefore not about absolute clarity.

It is about the relationship between colour, translucency, inclusions, cutting and light — all coming together to produce something that appears almost impossible: a star that seems to float across a gemstone.

And unlike the brilliance of a faceted stone, which is immediately apparent, the beauty of a star sapphire asks you to move it, turn it and watch.

Sometimes the most interesting gemstones are the ones that reveal themselves slowly.

Contact us to try on star sapphire rings.