16/06/2026
Star diopside and the phenomenon of asterism ✨
Among the many optical effects observed in gemstones, asterism is one of the most distinctive. This phenomenon occurs when light is reflected from oriented inclusions within a gemstone, producing a star-shaped pattern that appears to glide across the surface as either the stone or the light source is moved.
The gemstone featured here is star diopside, a variety of diopside that typically exhibits a four-rayed star, unlike the six-rayed asterism more commonly associated with corundum varieties such as star sapphire and star ruby.
The asterism observed in star diopside results from dense concentrations of microscopic, needle-like inclusions, commonly magnetite, that are aligned along crystallographic directions within the host mineral. When a strong, focused light source is directed onto the surface, these inclusions reflect light in a manner that produces the characteristic four-rayed star.
To display asterism effectively, the rough must be carefully oriented before cutting so that the inclusions are positioned correctly relative to the dome of the finished gemstone. A cabochon cut provides the curved surface necessary for the reflected rays to converge and form a visible star.
If the material were faceted, the effect would be significantly reduced or lost altogether.
📚 If you’re interested in finding out more about optical effects in gemstones, now’s the perfect time to explore our upcoming Gemmology Foundation course, which starts in September: https://gem-a.com/gemmology-courses/
📸 Gabriel Kleinberg
