Silk is the gemmological term for the fine, needle-like inclusions of rutile (titanium dioxide, TiO₂) aligned along crystallographic directions in corundum, where they appear under magnification as soft, milky, hazy fibre or veil-like patterns. The term is by analogy with the visual character of woven silk under indirect light. Silk is one of the most diagnostically important inclusion features in ruby and sapphire: its presence, condition, and orientation indicate origin character, contribute to optical phenomena including asterism and chatoyancy, and are the principal microscopic evidence in the determination of unheated status.

Origin and structure

Silk in corundum forms by exsolution of titanium oxide during the slow cooling of the host crystal from the high temperatures at which titanium is soluble in the corundum lattice. The exsolved rutile crystallises along three principal crystallographic directions in the corundum structure, producing the characteristic three-set hexagonal pattern visible when the silk is dense enough to support a star. In the metamorphic deposits of Burma (Mogok), Sri Lanka, and Madagascar, silk formation is widespread; in the basaltic-association deposits of Australia and Thailand, silk is rarer because the cooling history is different.

Asterism and chatoyancy

When the density of silk is high and the orientation is uniform, the resulting scattering produces star asterism — the six-rayed star of star ruby and star sapphire, cut as a cabochon to align the star directly above the dome. Twelve-rayed stars occur where two generations of silk are present in slightly different orientations. Cat's-eye chatoyancy in corundum, less common than in chrysoberyl or quartz, also depends on aligned silk-density inclusions. Asterism and chatoyancy require both the inclusion density and the cabochon orientation; faceted material with comparable silk would not show the phenomenon.

Silk as a heat indicator

Silk dissolves at the temperatures used in standard heat treatment of corundum — typically above 1400 °C for the basaltic-association ruby and sapphire, somewhat lower for Sri Lankan blue sapphire. Heat-treated stones therefore show partial or complete absence of silk, residual 'discoid' or melted-needle features where partial dissolution has occurred, and characteristic healed-fracture features. The presence of intact, undissolved silk in a corundum is the principal microscopic evidence of unheated status, and laboratories build their unheated determination around this and related observations. Heat indicators include altered silk, healed fissures with internal residue, and characteristic colour-zoning patterns.

Effect on appearance and value

Moderate silk in transparent corundum can support and even enhance value. Mild silk produces the soft 'velvet' character of fine Kashmir sapphire and the diffused, almost-glowing appearance of the best Mogok rubies. Excessive silk reduces transparency and clouds the body of the stone, and most heat treatment is undertaken specifically to reduce silk and clarify the apparent body. The trade convention is that the visual effect on the stone determines value contribution, with diagnostic implications for unheated status running alongside.

In the trade

For practitioners working with high-value corundum, silk is one of the principal microscopic features to examine and to discuss in the laboratory-report context. Unheated determinations on rubies and sapphires from Burma, Kashmir, and Sri Lanka command substantial market premiums, and the silk feature is the principal microscopic basis for those determinations. We treat silk observation as an essential competence in coloured-stone gemmology and recommend laboratory examination by Lotus Gemology, Gübelin, SSEF, AGL, or GIA for any consequential origin and treatment determinations on the upper-tier corundum.

Further reading

• GIA Gems & Gemology — corundum inclusion articles
• International Gem Society — ruby and sapphire reference
• Lotus Gemology — heat-treatment and silk identification notes