Star Sapphires

Star Sapphires are an interesting addition to the world of corundum gemstones. They differ from conventional sapphire gemstones owing to the presence of a feature called asterism. Asterism is a phenomenon in gemstones, notably in sapphires, that creates the visual imprint of vertical and/or horizontal lines that cut across the surface of the gemstone. A feature, when it is strongly present and is highly valued by jewellers and customers alike. Star sapphires are relatively rare and hence command a higher market value, however, their value is determined by customer demand, as many persons prefer to choose to go for conventional gemstone types. However, the structural anomalies that give rise to the distinguishing features of star sapphires have made them a special category in the gemstone market, that are sought out by customers who seek something out of the ordinary. Here we will explain the key characteristics of star sapphires, and in the process get into some of the terminologies, and provide information on the rare star sapphire type: black star sapphire, and provide a general guide when it comes to buying star sapphire jewellery. 

Distinguishing Characteristics of Star Sapphires

Star sapphires differ from conventional sapphire stones owing to the presence of structural distinctions that give rise to their uniqueness. An important step in identifying star sapphires, and later in determining their value, for not all-star sapphires, are made equal, is to understand the key features that distinguish them.

Asterism

Asterism is the defining feature of star sapphires. Asterism is a feature where inclusions in the gemstone are present in a particular manner, that gives it a distinct design. Found as two or three intersecting lines on the surface of the gemstone. In understanding the nature of asterism, one must understand the role of inclusions. Inclusions are materials or elements which are trapped within the crystal structure of the gemstone; that have entered the mineral in the process of its formation. Inclusions in gemstones are generally viewed as undesirable, as they disrupt the uniformity of the gemstone in terms of its colour and clarity. The key gemstone quality factor of clarity-one of the 4Cs-places a premium on gemstones that do not feature such inclusions, with gemstones with fewer and/or smaller inclusion being valued more in relation to other factors that make up the 4Cs. When it comes to star sapphires, the differences lie in the formation of these inclusions and their underlying character. This takes us deeper into the study of asterism in precious gemstones in relation to the feature known as rutile.

Rutile

Rutiles are minerals made primarily of titanium oxide (TiO₂): a metal element, that is found in a wide variety of rock types across the world. Given their prevalence notably in sedimentary rocks and the fact that minerals structures like gemstones are closely connected to such rock formations from which they emerge, this metallic ore mineral is prevalent in many gemstones. Since gemstones like sapphire and rubies form over the course of millions of years, the prevalence of these metallic elements manifest as distinct lines in the gemstone. It is important to note that titanium oxide in gemstones, in addition to the popular rutile, come in different types: anatase and brookite. Whilst the presence of these two other variants of titanium oxide produces variegated effects, it is the rutile types that stands apart. Whenever this metal element in the rutile type, is present in higher levels, and importantly in a distinct shape, such as intersecting lines; it gives rise to the geological feature known as asterism. Rutiles are often found in a variety of shapes in the gemstone, with certain gemstones being covered in these metallic lines. When it comes to star sapphires the key distinction lies in the apparent order in which these rutiles are present. As rutile being fewer, and limited to specific areas of the gemstone, cutting across it like intersecting lines producing a star-like effect, giving rise to the famous star sapphire.

Cabochon Cut

A key aspect of sapphires that feature asterism is the cut of the gemstone. Star sapphires are generally not subject to the traditional cutting processes of other gemstones: such as popular gemstone cuts like princess, round, emerald etc. Such cuts are reserved for sapphires and other gemstones where the presence of inclusions is viewed as a negative. Star sapphires where the inclusions are valued, the need for light to reflect on the clean surface is key. Whilst the physics of light and how it interacts with the complex crystal structure of the gemstone is best reserved for a science paper, what the reader needs to understand is that the smooth unfaceted surface of the gemstone is key in ensuring that the light is able to bounce off the gemstone and accentuate its star-like effect produced by the orderly rutiles. Now the cabochon cut is not solely reserved for star sapphires, however, this is the gemstone type that is best suited to be accentuated this distinct gemstone cut. 

Where are Star Sapphires Found?

When it comes to the top destination for star sapphire gemstones, the place also happens to be one of the top destinations for sapphire in general: Which is the island of gems Ceylon (Sri Lanka). Sri Lanka is one of the top sources for star sapphires, whose supply is built on the gemstone resource strength of the island nation. A country that is renowned for its famous blue sapphires and the elusive Padparadscha sapphire. Since the supply of star sapphires in the market is limited, it is best to seek out a source that has a reputation when it comes to supplying quality gemstones, and Ceylon sapphires with a strong position in the industry is a solid choice. It would probably not come as a surprise that the largest star sapphire, weighing over 500 kilos, at an astonishing 2.5 million carats, was found in the backyard of a home in Ratnapura: the famous city of gems. However, star sapphires have been unearthed in other parts of the world, with gemstone producing nations like Burma entering the mix. With the rarer black star sapphire being sourced predominantly in countries like Thailand.

What to Look for When Buying a Star Sapphire

The basic feature of a star sapphire is the presence of rutiles that are cut across the gemstone in two or more lines, producing a star-like effect. The quality of the star sapphire stands to be higher if the lines are clearly visible, and where the points of intersection are uniform. With the ideal number of rutile lines of three, that cut across the gemstone creating a star-like effect at its centre. Some star sapphires only contain two lines that intersect at the centre of the gemstone. Arguably the rarest variant in the star sapphire range is those which feature 12 lines: referred to as twelve ray or twelve-point star sapphires, which feature six intersecting lines that add another level to the star-like effect of the gemstone. However, when to comes to such gemstones, there is a possible trade-off, as the higher number of intersecting lines (i.e. rutiles) could mar the colour of the gemstone, with the white lines becoming too dominant on its surface. This phenomenon is rarer but is a feature that is often associated with a certain type of sapphire gemstone that carries the same feature: the black star sapphire. When it comes to purchasing a quality star sapphire, in addition to the central feature of star shape rutile structure, the basic principles of determining sapphire quality remain the carat, cut, and colour. With greater transparency of the stone that is associated with a higher value, in addition to the number of lines. However, these must be viewed in relation to the distinct appeal of this particular gemstone, notably its clarity.

Black Star Sapphire

It is worth noting that star sapphires are found in a variety of colours. Whilst blue is still the most popular option, star sapphires can be found in pink, yellow and even shades of purple. When it comes to black star sapphires, which are valued more for their rarer colour, and underlying it their different mineral character. Black star sapphires are found even in countries like the United States. What sets black sapphires apart is that unlike other star sapphires the factors contributing to asterism in the gemstone is not due to effects produced by rutiles, but via a complex geological process known as exsolution: or in layman's terms, the unmixing of a mineral that is composed of two or more purer elements. In the case of the black star sapphire, this involves the exsolution of the iron oxide hematite Fe₂O₃. 

Star Sapphire Rings

Getting hold of quality a star sapphire may seem like a tough task for many, so the task of buying a quality star sapphire ring is bound to be tricker. Whilst star sapphires have been on the market for a while, and jewellers who specialise in sapphires make it a point to showcase their offerings. When it comes to purchasing star sapphire rings, especially in fine jewellery applications, like engagement rings, the decision making needs to be more precise. As noted, star sapphires are rare in the market for gemstones, so when it comes to picking one, the reputation and history of the jeweller is a key factor. Given its rarity, and the relatively lower demand for star sapphire based jewellery, the most realistic way to go about getting hold of a star sapphire ring would be to consider the custom jewellery route. If you happen to have a solid star sapphire laying around, it would be a good idea to get it checked and certified and then seek out a jeweller who has the expertise in making custom made rings, particular those who can work with cabochon cut gemstones. In the industry, there is a certain trend when it comes to such exotic stones like star sapphires, where people hunt down the gemstones and then proceed to have it placed into a quality jewel: like a star sapphire engagement ring. So in practice is always best to start with locating a good star sapphire gemstone.