Silk & Needle Inclusions

Silk refers to fine, needle-like inclusions of rutile (TiO₂) that exsolve from the
corundum lattice during cooling, orienting along the crystal's three equivalent
directions to intersect at 60°. In sufficient density they scatter light and produce
the characteristic silky sheen that gives the feature its name; when the needles are
perfectly oriented and the stone is cut en cabochon perpendicular to the c-axis, the
three sets combine to create a sharp six-rayed star. Silk is diagnostically important
for two reasons: its morphology varies with geographic origin (short, fine needles
indicate Burmese provenance, longer needles at 60° characterise Sri Lankan material,
and dense networks are typical of Thai and Cambodian stones ^[1]),
and its condition signals heat treatment. Above 1200 °C needles dissolve inward
to produce "dotted" trails; above 1400 °C titanium re-enters the lattice entirely,
leaving a cleaner but depleted stone ^[2]. Intact, sharp
silk at 40× is therefore among the most compelling evidence that a corundum has
not been heat-treated.

Rutile silk is the defining internal feature of many natural corundums ^[2].
TiO₂ crystals exsolved from the corundum lattice during cooling, oriented at 60° intersections:

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 central to origin determination in corundum.
Short, fine needles indicate Burmese origin; longer needles at 60° indicate Sri Lankan origin
^[1].

Dense, properly oriented silk creates star effects (asterism). Three sets of rutile
needles at 60° create a six-ray star ^[2]:

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 ^{[2] [3]}
• Complete absence may indicate treatment or origin