Star Sapphire rutile needle orientation: centering 6-ray asterism during pre-forming?
5 Answers from travellers
Cutting asteriated corundum (star sapphires and star rubies) requires orienting the optic c-axis perpendicular to the base of the cabochon. Asterism is created by light reflecting off three intersecting sets of microscopic rutile (silk) needles arranged at 60-degree angles along the basal pinacoid plane. Using a polariscope or immersion cell under cross-polarized light, the lapidary identifies the optic axis by locating the circular optic interference figure (isogyres). The base of the pre-form is then ground parallel to this plane; if the c-axis is tilted even 5 degrees, the star will drift off-center.
A rounded, domed cabochon height is necessary to concentrate the reflected rays; shallow-topped cabochons produce weak, diffuse stars that disappear outside direct sunlight.
Beware of synthetic star sapphires (such as Linde stars) in street markets; natural Sri Lankan stones display slightly uneven rays and visible internal silk zones under back-lighting.
A pinpoint single-source penlight is the only reliable light source for grading asterism; multiple ceiling LEDs split the star into confusing secondary rays.
Export shipments of untreated natural star sapphires require standard valuation sealing through the National Gem and Jewellery Authority before departure.
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