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Asterism (gemmology)
Encyclopedia
Cabochon
A cabochon , from the Middle French caboche , is a gemstone which has been shaped and polished as opposed to faceted. The resulting form is usually a convex top with a flat bottom. Cutting en cabochon is usually applied to opaque gems, while faceting is usually applied to transparent stones...
. The typical asteria is the star-sapphire
Sapphire
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...
, generally a bluish-grey corundum
Corundum
Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...
, milky or opalescent, with a star of six rays. In red corundum the stellate reflexion is less common, and hence the star-ruby
Ruby
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...
occasionally found with the star-sapphire in Ceylon is among the most valued of "fancy stones". When the radiation is shown by yellow corundum, the stone is called star-topaz
Topaz
Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al2SiO42. Topaz crystallizes in the orthorhombic system and its crystals are mostly prismatic terminated by pyramidal and other faces.-Color and varieties:...
. Cymophane, the chatoyant chrysoberyl
Chrysoberyl
The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula BeAl2O4. The name chrysoberyl is derived from the Greek words χρυσός chrysos and βήρυλλος beryllos, meaning "a gold-white spar". Despite the similarity of their names, chrysoberyl and beryl are two completely...
known as cat's eye, may also be asteriated. In all these cases the asterism is due to the reflexion of light from twin-lamellae or from fine tubular cavities or thin enclosures definitely arranged in the stone. The astrion of Pliny the Elder
Pliny the Elder
Gaius Plinius Secundus , better known as Pliny the Elder, was a Roman author, naturalist, and natural philosopher, as well as naval and army commander of the early Roman Empire, and personal friend of the emperor Vespasian...
is believed to have been a moonstone
Moonstone (gemstone)
Moonstone is a sodium potassium aluminium silicate, with the chemical formula AlSi3O8.-Etymology:Its name is derived from a visual effect, or sheen, caused by light reflecting internally in the moonstone from layer inclusion of different feldspars....
, since it is described as a colourless stone from India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
having within it the appearance of a star shining with the light of the moon. Star-stones were formerly regarded with much superstition.
Description
An asterism is an optical phenomenonOptical phenomenon
An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....
displayed by some rubies
Ruby
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...
, sapphire
Sapphire
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...
s, and other gems
Gemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...
(i.e. star garnet, star diopside
Diopside
Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the monoclinic prismatic class...
, star spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
, etc.) of an enhanced reflective area in the shape of a "star" on the surface of a cabochon
Cabochon
A cabochon , from the Middle French caboche , is a gemstone which has been shaped and polished as opposed to faceted. The resulting form is usually a convex top with a flat bottom. Cutting en cabochon is usually applied to opaque gems, while faceting is usually applied to transparent stones...
cut from the stone. Star sapphires and rubies get their asterism from the titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
impurities (rutile
Rutile
Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
) present in them. The Star-effect or "asterism" is caused by the dense inclusions of tiny fibers of rutile (also known as "silk"). The stars are caused by the light reflecting from needle-like inclusions of rutile aligned perpendicular to the rays of the star. However, since rutile is always present in star gemstones, they are almost never completely transparent.
A distinction can be made between two types of asterism:
- Epiasterism, such as that seen in sapphire and most other gems, is the result of a reflection of light on parallel arranged inclusions inside the gemstone.
- Diasterism, such as that seen in rose quartz, is the result of light transmitted through the stone. In order to see this effect, the stone must be illuminated from behind.