Asterism (Star Effect)
Asterism in gemstones including star ruby, star sapphire, and other star stones with formation, quality factors, and identification.
Introduction
Asterism is the star-shaped pattern of concentrated light that appears on the
domed surface of a correctly cut cabochon when held under a point light source.
It is caused by reflection from two or more sets of parallel needle-like
inclusions, most commonly rutile (TiO₂) silk, oriented along crystallographic
axes within the host mineral. [1]
Each set of needles reflects light along a single axis, producing one bright ray;
multiple intersecting sets produce multiple rays. Trigonal corundum hosts three
sets of rutile needles at 120° to one another, generating the classic six-ray
star of star ruby and star sapphire. The stone must be cut with its base parallel
to the plane of the needles and a sufficiently high dome; poor orientation shifts
the star off-centre or destroys it entirely. Star rubies from Mogok, Burma, and
star sapphires from Sri Lanka are the most prized examples, though asterism also
occurs in garnet, diopside, quartz, and rose quartz. [2][3]
How Stars Form
The formation of asterism requires specific conditions:
Requirements
- Needle inclusions oriented along specific crystal directions
- Light reflects from each set of needles, creating a bright band
- Multiple intersecting needle orientations create multiple rays
- The stone must be cut en cabochon with base parallel to needle planes
- Proper orientation during cutting is critical for centred stars
Types of Stars
| Type | Ray Count | Cause | Examples |
|---|---|---|---|
| Six-ray star | 6 | Three sets of needles at 60° | Star ruby, star sapphire |
| Four-ray star | 4 | Two sets of needles at 90° | Star diopside, some garnets |
| Twelve-ray star | 12 | Two overlapping 6-ray patterns | Rare star corundum |
Star Ruby and Star Sapphire
Corundum is the most important source of star gems:
The Silk
Sources
| Origin | Characteristics |
|---|---|
| Myanmar (Mogok) | Fine red body; sharp stars |
| Sri Lanka | Good stars; various colours |
| India | Black star sapphire |
| Thailand | Dark body; often heated |
| Madagascar | Various qualities |
Star Quality Assessment
High Quality Stars
- Sharp, well-defined rays
- Complete rays from girdle to girdle
- Centred on cabochon apex
- Smooth, fluid movement
- Attractive body colour
- Good dome height
Low Quality Stars
- Fuzzy or diffuse rays
- Incomplete or broken rays
- Off-centre star position
- Jerky or patchy movement
- Poor body colour
- Flat or overly high dome
Synthetic and Treated Stars
Other Star Gems
| Gem | Star Type | Characteristics |
|---|---|---|
| Star diopside | 4-ray | Black body; distinct white cross |
| Star garnet | 4 or 6-ray | Almandine; Idaho famous source |
| Star quartz | 6-ray | Rose quartz; often weak |
| Star spinel | 6-ray | Rare; collector interest |
| Star sunstone | 4-ray | Oregon copper sunstone |
Cutting Considerations
Proper cutting determines star quality:
- Base orientation: Parallel to needle plane
- Dome height: Adequate dome shows star better
- Centering: Star should be centred on dome apex
- Symmetry: Even cabochon shape
- Surface quality: Good polish without scratches
References
- ↑ 1. Read, P. (2014). Gemmology (3rd ed.). Butterworth-Heinemann/Routledge. DOI: 10.4324/9780080507224.
- ↑ 2. Keller, P. (1983). The Rubies of Burma: A Review of the Mogok Stone Tract. Gems & Gemology, 19(4), 209–219. DOI: 10.5741/gems.19.4.209.
- ↑ 3. Schumann, W. (2009). Gemstones of the World (4th ed.). Sterling. ISBN: 978-1-4027-6829-3.