Star gemstones are loved by collectors of the unusual. Whereas the beauty of conventional gemstones is based upon the absence of inclusions, star gemstones depend on inclusions to achieve their unique look and feel.
As these special crystals form, natural cavities or fine needle-like inclusions grow parallel to more than one crystal face. Once formed, proper cutting of the crystal is critical to achieving the desired star effect.
Just as a sculptor carves a block of marble to reveal the masterpiece within, the lapidary will skilfully orient, cut, and polish an asteriated crystal until achieving a perfectly smooth dome with a flat or convex base – a cabochon. The apex of the dome must be perpendicular to the plane of inclusions for light to reflect from them and produce a star effect.
The crystallography of a gemstone determines the type and quality of star and can provide vital clues about its identity. For instance, an ultramarine blue cabochon with a hexagonal spiderweb-like silk structure and a three-armed star crossed at 60 degrees indicates trigonal symmetry, typical of sapphire.
These “silk” inclusions are densely-packed needles of rutile (titanium dioxide). If a second set of rutile needles is oriented, a 12-rayed star may result; however, this is rare. More commonly encountered are golden black star sapphires that exhibit a six-rayed star due to included, oriented crystals of hematite.
A wavy four-rayed star on a bewitching dark green or black opaque body is a telltale sign of diopside, reflecting its lopsided symmetry and needle-shaped inclusions of magnetite.
The finest and most popular star gemstones are ruby and sapphire from the corundum family; however, other species are available. Star diopside is less common, while star chrysoberyl and enstatite are rare. Gemstones that may occur with a weak star effect include spinel, beryl, garnet and citrine.
While epiasterism depends on light reflected from inclusions to produce a star, diasterism is a star effect seen in transmitted light, as observed in rose quartz.
The most valuable star gemstones combine a strong, sharp and centred star with an intense, highly saturated and even body colour. The star should ideally have long, straight arms that extend to the edges of the cabochon. Star gemstones should be well proportioned with smooth domes of even symmetry; however, lapidaries may produce lumpy or asymmetrical gemstones to bring out the star to best effect.
A star effect may be induced in natural or synthetic non-star corundum by controlled heat treatment or through surface diffusion treatment using a combination of heat and chemical elements. Synthetic sapphire stars have a very sharp and unnatural looking star that appears too perfect.
Star gemstones love direct light and are displayed to their best advantage in jewellery that is fully exposed to it, such as rings. Less durable or softer star gemstones are recommended for pieces like pendants or earrings.