Sheen is the effect caused by reflection of light from below the surface of the gemstone (lustre is on the surface).
There are several terms associated with sheen (listed below) and the causes can range from inclusions, internal structures and interference.
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, fibres 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.
Famous examples of this type of sheen are Chrysoberyl Cat's-Eye (or "Cymophane") and Tiger's-Eye (Quartz with asbestos fibres).
Others gemstones that may show chatoyancy are:
Asterism is, like chatoyancy, caused by reflection on inclusions. However the inclusions are aranged in different directions causing several lightstreaks on the surface of the en-cabochon stone.
There can be 4-pointed, 6-pointed and 12-pointed stars.
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 epiasterim.
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.
Star-Emeralds 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.
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)