Sheen is the effect caused by reflection of light from below the surface of the gemstone (luster is on the surface).
There are several terms associated with sheen (listed below) and the causes can range from inclusions, internal structures and interference, reflection and refraction.
This type of sheen occurs on gemstones with parallel orientated inclusions, like fibers, needles and hollow tubes. The effect of the reflection on these parallel arranged needles, fibers or hollow tubes is a light streak which runs at a 90° angle over the inclusions.
In order for this effect to be seen, the gemstone needs to be cut en-cabochon.
Naming these types of gemstones is generally done with the suffix Cat's-Eye, as in Alexandrite Cat's-Eye.
Asterism is, like chatoyancy, caused by reflection on inclusions. However the inclusions are arranged in different directions causing several streaks of light on the surface of the en-cabochon stone.
In general, the inclusions which cause the stars are orientated parallel to the crystal faces.
In Corundum you may find (usually in Thai sapphire) a 12-pointed star due to inclusions (rutile and hematite) following both the 1st order and the 2nd order prism.
This type of asterism (due to reflected light) is named epiasterism.
Diasterism is asterism caused by transmitted light (from behind the stone) and can be seen in some Rose Quartz and Almandine Garnet.
Garnet may produce 4-pointed stars which intersect at 90°, whilst in Diopside the 4-pointed stars intersect at 73°.
Corundum usually forms 6-pointed stars (mostly due to rutile or hematite needles).
Synthetic corundum may also show asterism. Usually the stars are much better defined than their natural counterparts.
Emerald has been reported to show a 6-pointed star.
Some stones, especially corundum, have orientated inclusions but in insufficient quantities to show a star.
These stones are generally facetted and occasionally you may see light reflected from small groups of such inclusions. This is termed Silk.
We describe stones which show asterism with the prefix Star as in Star-Emerald.
Some gemstones that may show asterism:
- Ruby (6-pointed)
- Sapphire (6-pointed, rarely 12-pointed)
- Rose Quartz (6-pointed)
- Spinel (4 or 6-pointed)
- Garnet (4 or 6-pointed)
- Diopside (4-pointed)
Iridescence is the play of color, or a series of colors, produced by interference or diffraction (or both), either when light is reflected from thin films (inclusions), twinning planes or from the unique structure of precious opal.
There are several types of iridescence that have their own particular causes:
- Adularescence (or Schiller)
Labradorescence is the effect seen in Labradorite (a Feldspar) and Spectrolite (a Labradorite found in Finland). It is caused by interference on the boundaries of lamellar twin planes, which are usual in Feldspars.
Many Labradorites are carved to exploit this unique type of sheen.
This type of sheen is exhibited in Moonstone (another Feldspar) and caused by reflection on the lamellar twinning planes.
The result is a blue color floating just below the surface of the stone. This is also named Schiller.
Aventurescence is named after Aventurine Feldspar, which is also known as Sunstone due to the play of color that is caused by reflection on tiny and thin inclusions of goethite and hematite (or both). Giving the stone a golden or reddish-brown color and specular reflections.
The causes of play of color in Opal were long uncertain until the invention of the electron microscope. This enabled scientists to see the unique structure of Opal at high magnification.
It was discovered that Opal is made up of small spheres of silica.
The interference occurs when part of the light gets reflected from the surface of a sphere and another part gets refracted inside the silica sphere, being reflected again.
Diffraction in Opal is the result of light hitting a gap between the spheres and then being split up into its spectral components. In precious opal the larger spheres, about 350 µm (micro meters) in diameter gives the red flashes with changes in viewing angles. Smaller spheres result in green, blue or purple flashes which cannot increase in wavelength to give a red flash. Therefore, the sizes of the 'gaps', or 'voids' determine which color is seen.
This is the same as what happens with diffraction grating material of which some spectroscopes are made.