‘Silk’: a descriptive term of the appearance (for its similarity) adopted to illustrate, in Ruby and Sapphire corundums, the peculiar configuration assumed by the acicular (needle-like) crystalline Rutile inclusions, which, arranged in three directions parallel to the three pairs of faces of the hexagonal prism, intersect according to 120° (and 60°) angles lying in planes perpendicular to the main axis of the crystal. Such a system of inclusions, when extended to the entire equatorial section of the original (cabochon-cut) crystal, gives rise to the optical phenomenon of Asterism.
