The 40.57 ct transparent-to-semitransparent yellow oval tablet shown here was submitted to the GIA Laboratory in New York as a "star sapphire." Standard gemological testing identified it as a natural sapphire. What caught our eye immediately, though, was that the starlike appearance was not asterism, but rather a "trapiche" pattern—a fixed, reflective six-armed star with associated color zoning.

While trapiche patterns are common in emeralds, they are less common in corundum, and are typically restricted to ruby. Trapiche sapphire is considerably rarer, and most is blue to gray; an extensive literature search with the help of staff members at GIA's Richard T. Liddicoat Gemological Library and Information Center did not reveal any published accounts of yellow trapiche sapphire.

Exposure to long-wave UV radiation caused this stone to fluoresce weak-to-medium orange in the triangular growth sectors, while the "arms" fluoresced weak orange or were inert; the short-wave reaction was weaker. Fluorescent chromium lines at approximately 680 nm and an absorption line at 450 nmin the desk-model spectroscope indicated the presence of trace amounts of chromium and iron, respectively.

A small core area parallel to the c-axis formed the center of the star formation. Reflected light best revealed the inclusions. The triangular growth sectors had a yellow bodycolor and were less included—with mostly fluid remnant "fingerprints," two-phase inclusions and reflective films—than the radiating spokes. The arms were nearly colorless and to the unaided eye appeared to contain abundant "needles." With magnification, these needles could be resolved into narrow ribbons of reflective thin-film inclusions in planes that were approximately perpendicular to the core or c-axis. Metallic, small dark, or irregular transparent crystals were present in both the arms and triangular growth sectors.

These differences in inclusions, color and growth structure suggested that the sapphire crystallized from its core outward, with the sectors and arms probably growing concurrently but at different rates. Note that the definition of trapiche varies, with some referring to it as skeletal growth resulting in inclusions trapped in the arms of the star, or the arms being composed of a different material than the host. Other definitions include preferential exsolution of inclusions in the dendritic and growth sectors, or structure- and growth-related chemical variations that result in color-causing mechanisms differentiating the arms.

Although many of the reported trapiche corundums—rubies in particular—have arms composed of distinct mineral phases, this stone was composed primarily of corundum throughout. Even so, the GIA Lab categorized this sapphire as trapiche due to its distinct core, inclusion pattern and growth structure. It is the first yellow trapiche sapphire we have seen.

Donna Beaton
GIA Laboratory, New York