Silk & Needle Inclusions

Silk refers to fine, needle-like inclusions that create a silky lustre in gemstones.
Most commonly composed of rutile (TiO2) in corundum, silk is one of the most
important diagnostic features for both identification and origin determination.

Rutile silk is the defining internal feature of many natural corundums:

Composition:

• Titanium dioxide (TiO2) crystals
• Exsolved from corundum lattice during cooling
• Oriented along crystallographic directions (60° intersections)

Appearance:

• Fine, needle-like crystals
• Often intersecting at 60° angles
• Creates silky sheen when dense
• Length and density vary by origin

The characteristics of rutile silk are crucial for origin determination in corundum:

Dense, properly oriented silk creates star effects (asterism):

Requirements for asterism:

• Sufficient density of silk needles
• Proper orientation (intersecting at 60°)
• Correct cutting (cabochon, oriented perpendicular to c-axis)

Star types:

• 6-ray star - Three sets of silk at 60° (common in corundum)
• 12-ray star - Six sets of needles (rare, exceptional)
• 4-ray star - Two sets at 90° (in garnets, diopside)

Heat treatment significantly affects silk appearance:

Low to moderate heat:

• Silk may partially dissolve
• Needles become interrupted ("dotted silk")
• Stone clarity improves

High heat:

• Silk completely dissolves
• Titanium re-enters corundum lattice
• Stone becomes cleaner but loses silk

Diagnostic value:

• Intact silk suggests no high-temperature treatment
• Dissolved/dotted silk indicates heat treatment
• Complete absence may indicate treatment or origin