| In the gemstone world there are a special group | | | | clarity and distinctness of the star. Generally the |
| of gems known as phenomenal gemstones. | | | | star must be evaluated using a single source light |
| These are gems which exhibit special optical | | | | such as a penlight; it is usually very difficult to see |
| phenomena, including asterism (the star effect), | | | | the star under diffused light. Moving the light back |
| chatoyancy (the cat's eye effect), adularescence | | | | and forth should cause the star to move across |
| (shimmering light) and iridescence (the rainbow | | | | the stone's surface. The rays of the star should |
| effect). | | | | be evenly distributed and the rays should be |
| Since these phenomenal gems are fairly rare, | | | | relatively straight. The most common stars are |
| they naturally attract the attention of collectors. | | | | 4-ray and 6-ray, though 12-ray stars have been |
| In some cases all or most of the gems belonging | | | | seen. |
| to a particular variety display the phenomenon, in | | | | Color is also very important in a star gemstone. |
| the way that most moonstone exhibit | | | | Ideally the color should be equivalent to a non-star |
| adularesence. But in other cases the optical effect | | | | specimen of the gem, but the presence of rutile |
| is found only occasionally in a particular type of | | | | in the stone tends to weaken the color. So in the |
| gem, and it is the rare cases that are of special | | | | best case there is enough rutile to create a |
| interest to collectors. | | | | distinct star while minimizing the effect on the |
| Asterism or the star effect is an example of an | | | | color. |
| optical effect found very rarely in a small number | | | | Burma and Sri Lanka are the most important |
| of gem types cut as cabochons. The most | | | | sources for star ruby and star sapphire, though |
| famous examples are star sapphire and star ruby. | | | | Thailand is famous for the black and gold star |
| But asterism may also be exhibited by | | | | sapphires found only in Chanthaburi province. |
| moonstone, quartz, spinel, citrine, diopside, | | | | Some star sapphires are produced using a |
| emerald, garnet and chrysoberyl. | | | | diffusion treatment, where the sapphire is heated |
| The reason that asterism is so rare is that it | | | | with a coating of titanium dioxide which diffuses |
| requires the gem to have inclusions of rutile | | | | into the corundum and creates rutile needles. |
| needles, and for the needles to be aligned in such | | | | Synthetic star sapphires were first produced in |
| a way as to reflect light in a multi-ray pattern | | | | the late 1940's by Linde, a division of Union |
| forming a star. The usual rule of thumb in | | | | Carbide. They are currently produced in Japan by |
| gemstones is the fewer inclusions the better. But | | | | Nakazumi Earth Crystals. The synthetic gems |
| this is a case where the value is enhanced by the | | | | tend to have perfect color and star so vivid and |
| right sort of inclusions. | | | | straight that it appears painted on the stone. |
| The value of a star gemstone depends on the | | | | |