Silk

Fine needle-like inclusions, usually rutile, laid along the crystal directions of corundum. Silk does three jobs at once. Intact silk argues against high-temperature heating, dissolved silk argues for it, and finely dispersed silk scatters light into the soft glow prized in Kashmir and Mogok sapphire. Dense oriented silk, cut en cabochon, makes the star. One feature, read three ways, which is why laboratories photograph it.