Cutting Process of Star Sapphires
Star Sapphires are always cut into Cabochons i.e. there are no facets on the gemstone. Since the star effect is caused due to the light reflecting off Rutile needles, hence sometimes the base of the cabochon is left rough and unpolished to decrease the light transmitted through the gemstone and increase the light reflected from it. This increased reflection makes the star effect more prominent. Such a method is more common for transparent to translucent stones than opaque ones.
The cutting and polishing of a Star Sapphire involves a lot of experience, precision and skill. The star has to be oriented correctly along the C-axis of the gemstone. In most cases, the axis is visible as the gemstone is longer or shorter in that direction. However, in stones which have been obtained from gem gravels, the crystallographic shape of Sapphire is not present as the edges have been rounded by river transportation over long distances. In such cases, the axis has to be found out before beginning to polish it. A simple method that can be used to find this is to check the rough sapphire under a polariscope with an interference sphere. However, this method is useful only if the gemstone is vitreous in lustre and transparent. After locating the axis, comes the hard part to carve out a cabochon from the rough whilst retaining maximum weight and maintaining optimum dimensions. After a rough shape has been made, the gemstone is polished on a polishing lap till a glass-like polish is attained. In case the sapphire is not properly oriented along the c-axis, the star would become off-cantered decreasing the value of the gemstone.