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CHAPTER V.
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STAR STONES.
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ERTAIN varieties of Corundum, especially the greyish-blue semi-transparent Sapphires when cut en cabochon, shew a star of light, more or less perfect, reflected from the convex surface. Such stones are therefore commonly called Star Stones, whilst by the Ancients they were designated Asterias. According to Plutarch, the River Sangaris produced a gem called Aster, which was luminous in the dark, and was known to the Phrygians as Ballen, or " The King." A gem called Asteriles, found
inside a huge fish called " Pan," from its resemblance to that god, is
also described by Ptolemy Hephaestion. The term Asteria has been used
by different authors in various senses at various times ; but there can
be no doubt that Pliny understood by it the same gem that we do now. A
purplish Star Sapphire was known to Pliny as the Ceraunia, or "Lightning-stone," and it was probably the same stone that was termed Astrapia.
The optical phenomenon presented by star-stones is known as Asterism, and
its cause is to be sought in the internal structure of the crystal ;
all the Star-Stones exhibiting a peculiar laminated texture, and
generally presenting, on the basal plane, a system of fine striations
related to the direction of the lines of light, which form by their
intersection the chatoyant star. In the Star Sapphires there seem to
be three sets of structural planes, the
edges of which intersect at angles of 6o°; and when a
Ο
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