In the gemstone world there are a special group of gems known as phenomenal gemstones. These are gems that exhibit special optical called an asterism or star effect. Asterisms are found very rarely in a small number of gem types cut as cabochons. The most famous examples are star sapphire and star ruby. But asterism may also be exhibited by a number of other gemstones, such as moonstone, quartz, spinel, citrine, diopside, emerald, garnet and chrysoberyl.
Color is also very important in a star gemstone. Ideally the color should be equivalent to a non-star specimen of the gem, but the presence of rutile in the stone tends to weaken the color. So ideally, there should be enough rutile to create a distinct star whilst maintaining good color.
Star sapphire can be distinguished from other forms of sapphire by its unique asterism. The six-rayed-star effect appears most clearly under natural light.
Star ruby can often be distinguished by its pleochroism, which means that the color varies with the viewing angle. The six-rayed-star effect shimmers best over the surface of the stone when moved in daylight.