Needle — The Long Slender Crystal Inclusion in Gemstones
Needle — The Long Slender Crystal Inclusion in Gemstones
Rutile, tourmaline, actinolite and other slender inclusions used in identification, origin, and asterism
A needle, in gemmological inclusion terminology, is a long slender crystal inclusion oriented along a specific direction within the host gem, typically aligned with one of the host's crystallographic axes. Needles are common in many gem species and are diagnostic features used in species identification, origin determination, and the optical phenomenon of asterism. The most familiar needles in commercial gemmology are the rutile needles in corundum (sapphire and ruby), where dense oriented rutile creates the silk that produces star sapphires and star rubies.
Composition and orientation
Needles in gemstones are most commonly composed of rutile (TiO2), but the term covers needle-like inclusions of various minerals: tourmaline needles in topaz and quartz, actinolite needles in nephrite, hornblende and amphibole needles in beryl, ilmenite and other ore needles in corundum, calcite needles in Burmese ruby, and apatite needles in some chrysoberyl. The orientation is typically parallel to the host's crystallographic axes — three sets at 60-degree intervals in the basal plane of corundum, for example, producing the six-rayed star pattern when the needles are dense enough.
Rutile silk and asterism
The most commercially significant needle inclusion is the rutile silk in corundum. Fine, dense, oriented rutile needles in sapphire and ruby produce a silvery-white sheen — the silk — that gives Burmese, Sri Lankan, and Kashmir corundum some of its characteristic optical character. When the silk is dense enough and well oriented, the stones display asterism: a star pattern visible on the polished cabochon surface, typically with six rays in standard star sapphire and star ruby, and rarely with twelve rays in stones with two intersecting sets of needles.
The presence and quality of rutile silk are also diagnostic for origin determination. Burmese ruby characteristically shows fine, dense, well-oriented rutile silk; Sri Lankan sapphire shows similar but often slightly different silk patterns; Kashmir sapphire shows a distinctive velvety silk that contributes to its famous bodycolour. Heat treatment of corundum, particularly at temperatures sufficient to dissolve rutile silk, removes or alters the inclusion population and is detectable as one of the diagnostic indicators of treatment in laboratory examination.
Identification value
Needle inclusions are diagnostic in several ways. The mineral identity of the needles can be determined by Raman spectroscopy and contributes to species and origin identification. The orientation of the needles relative to the host's crystal habit confirms the host species and reveals whether the stone is single-crystal, twin-crystal, or composite material. The density and pattern of needles vary between sources for the same species, providing one of the standard origin-determination criteria used by major laboratories. The Gübelin and GIA photoatlases of inclusions document needle inclusions extensively across all major gem species.
Treatment indicators
Needle inclusions also provide treatment indicators. Heat treatment at sufficient temperature dissolves rutile and other needle inclusions, producing characteristic remnant ghosts or partial-dissolution features visible under magnification. The detection of altered or dissolved needle inclusions is one of the standard diagnostic indicators that major laboratories use to identify heat treatment in corundum and other species. The trade convention for treatment disclosure depends in part on these microscopic observations.