Professional Jeweler Archive: Written in the Stars

August 2001

Gemstones & Pearls/Gemology


Written in the Stars

What causes asterism?


Few things can compare with the joy of closing a sale and wrapping up a sapphire ring for your favorite customer. But one of them is showing a customer the magic that occurs when you expose a star sapphire or ruby to direct light, such as a penlight.

Poof! A star appears in the gem, you both gaze intently and the customer without hesitation asks how it happens.

Do beads of perspiration decorate your brow as you grope for the right answer? Or do you remain cool because you know how to explain it?

Phenomenal, Isn’t It?

Some gemstones, often very rare ones, exhibit unusual optical personalities called phenomena. (Another example is the cat’s-eye phenomenon in gems such as chrysoberyl or tourmaline.)

Sapphires and rubies – both of which are gem varieties of the mineral corundum – have the star phenomenon when minute needles of the mineral rutile – sometimes called “silk” – are oriented inside them in the direction of the crystal growth system, which is hexagonal. There’s a relationship between the gem’s growth and the number of rays you see.

Optical Options

When needles are perpendicular – at right angles – to each other, you see the cat’s-eye effect. When needles intersect each other in multiple groups of three at 120&Mac251; angles throughout the gem, you see a six-rayed star or asterism (aster is Greek for star). Asterism is best seen when a gem is cut as a cabochon.

These complex notions are probably easiest to explain with diagrams (see p. 48). Asterism is most visible with direct light, such as a fiber-optic light, penlight or other single beam of light, including direct sunlight. With diffused illumination, the stars are not as distinct because less direct light reflects off of the rutile needles.

The quality and value of an asteriated gem is judged by:

  • The distinctiveness of its star.
  • The length and degree of straightness of each ray (straight rays, girdle-to-girdle are best).
  • The strength and uniformity of the gem’s color. Rubies are most valuable and must be defined as red (in tones of light to dark) or purple red. Pink and milky pink stones are considered to be pink sapphires.
  • The regularity of the gem’s outline.
  • The gem’s size.

Final Notes

When rutile needles are found in vast quantities, the sapphire or ruby is rendered opaque or translucent. When only small clouds of needles exist, asterism is visible in only a small area and it’s not particularly suitable as a phenomenal gem, if at all. These gems are often heat-treated to improve their clarity by dissolving some rutile needles. They become transparent and are more suitable for faceting.

It’s also possible to induce asterism by extending the heat treatment of some natural sapphires and rubies and by very gradual cooling, a process that allows rutile crystals to form.

– by Robert Weldon, G.G.

Asterism is best-known in sapphires, like these, and rubies, though it also occurs in quartz and spinel. Photo by Robert Weldon.
Figure A shows the three directions in which rutile needles form in corundum. The needles are parallel to the crystal faces (six faces in a hexagon). But because three crystal faces are parallel to three others, you count only three directions. Illustration by Orasa Weldon.
Figure B illustrates a multitude of juxtaposed needles and how they result in asteriation. The outline shows how the cabochon should be oriented and cut. Illustration by Orasa Weldon.
Figure C shows what you see when a direct light source reflects off of the surface of the oriented needles. Illustration by Orasa Weldon.
At 35X magnification rutile needles and their unique orientations are easy to discern. Photo by Robert Weldon.

Copyright © 2001 by Bond Communications