Pearl Luster
Pearl luster is the quality and quantity of light a pearl reflects from its surface or near surface. It is a complex effect, quite different from the luster of surface of polished gemstones. It is produced partly by the surface reflection of light and partly by a subtle iridescence. This iridescence is caused by:
- Diffraction at the edges of overlapping platy crystals of aragonite;
- Interference when light penetrates some of the thin plates, and is then reflected back to the surface.
Interference will take place on the surface of a pearl due to refraction and reflection of light on the thin alternating layers of aragonite and organic substance. This is a complicated optical phenomenon which occurs on a curved surface and may continue into deeper layers. This explains why individual pearls show different degrees of iridescence.
When natural pearls are graded, the factor “luster” is considered equal to the type of iridescence that is termed “orient”. It has different meaning in cultured pearls. The five grades given determine the strength of the reflection of the light source on the centre of the pearl. This means that sharpness and intensity of the reflection are determined.
- 1.Highest: strong luster of even distribution and distinctly perceived reflection of the light source;
- 2.Very High: Luster is less strong and less even, the perceived reflection of the light source is less distinct;
- 3.High: Luster is dull with only a distorted perceived reflection of the light source;
- 4.Good: Luster is dull with nearly no perceived reflection of the light source
- 5.Poor: Luster is distinctly dull with no perceived reflection of the light source.
A pearl with a thin nacre thickness can’t show a good luster. Although a pearl with a good nacre thickness need not necessarily show a good luster and may possible be placed into grade 3 or lower.
