---
title: "Asterism: meaning in gemology"
description: "Asterism is a star of light that glides across the dome of a gem. Light reflects from fine needles, often rutile, that lie in two or three directions."
url: https://digitalgemology.com/learn/glossary/asterism
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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[Glossary](https://digitalgemology.com/learn/glossary) · Gemstone properties

# Asterism

Also called star effect, star sapphire, star ruby

Asterism is a star of light that glides across the dome of a gem. Light reflects from fine needles, often rutile, that lie in two or three directions.

## In more detail

Star sapphires and star rubies usually show six rays, and occasionally twelve. Some other gems show four. The stone must be cut as a cabochon to show the star. The finest stars are sharp, straight and centered.

## On a grading report

Labs name the phenomenon, for example star sapphire.

## Related terms

- [Silk](https://digitalgemology.com/learn/glossary/silk): Silk is a mass of fine, crossing needle inclusions, usually rutile, in ruby and sapphire.
- [Needle](https://digitalgemology.com/learn/glossary/needle): A needle is a thin, elongated crystal inclusion that looks like a tiny rod at 10x.
- [Chatoyancy](https://digitalgemology.com/learn/glossary/chatoyancy): Chatoyancy is a single band of light that glides across a gem, like the slit eye of a cat. Light reflects from many aligned, needle-like inclusions.
- [Sapphire](https://digitalgemology.com/learn/glossary/sapphire): Sapphire is corundum of any color other than red. The word alone means a blue stone; other colors take a color word, such as yellow sapphire.
- [Ruby](https://digitalgemology.com/learn/glossary/ruby): Ruby is the red variety of the mineral corundum. Chromium causes its red color.

[All 328 terms](https://digitalgemology.com/learn/glossary)