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Natural star ruby asterism: titanium dioxide rutile needle intersections at 60 degrees?

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M
Marta Stones•910 rep

How do lapidaries cut rough Ratnapura star rubies into domed cabochons to align the three sets of rutile needles and produce a centered 6-ray star?

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6 Answers from travellers

✓Accepted Answer

Asterism in Sri Lankan star rubies is created by epiaxial networks of microscopic rutile (TiO2) needles oriented along the three crystallographic a-axes in the basal pinacoid plane, intersecting at exact 60-degree angles. To produce a centered six-ray star, the lapidary must orient the stone so that the optic c-axis is completely vertical and perpendicular to the flat base of the cabochon. When light enters the smooth, highly domed convex surface, it reflects off the three sets of silk needles, focusing three intersecting lines of light directly at the apex of the dome.

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D
Dr Kularatne•1950 rep

If the cabochon dome is cut too shallow or flat, the reflected rays become diffuse and fail to converge into sharp, well-defined rays.

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M
Marta Stones•910 rep

Examine the star under a single pinpoint light source (like the morning sun or a direct penlight); diffused fluorescent ceiling lights scatter the star into confusing reflections.

19
I
Ian Flavors•880 rep

Synthetic star rubies (such as Verneuil flame-fusion stones) display unnaturally sharp, perfect stars with zero internal growth silk visible under a 10x triplet.

16
E
Elena Granite•925 rep

Heat treatment designed to clarify rubies often dissolves delicate rutile silk entirely, destroying natural asterism permanently.

14
D
Dr Senaka Soil•1820 rep

Natural Ratnapura star rubies regularly feature a translucent, silky purplish-red body color with characteristic liquid fingerprint inclusions in the base.

12
S
Sarah Drone•940 rep

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