Asterism (Star Effect)
Asterism in gemstones including star ruby, star sapphire, and other star stones with formation, quality factors, and identification.
phenomena/asterism star-ruby star-sapphire silk
Introduction
Asterism is the appearance of a star pattern on the surface of a cabochon-cut gem.
It is caused by light reflecting from oriented needle-like inclusions (usually
rutile silk) arranged parallel to crystallographic directions.
Star rubies and star sapphires are the most important asteriated gems, though
stars also occur in garnet, diopside, quartz, and other minerals.
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
- Rutile (TiO₂): Tiny needle-like crystals
- Orientation: Forms along hexagonal crystal axes
- Three sets: At 120° angles to each other
- Creates: Classic six-ray star
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 is essential for 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