This is the cross chiastolite ring. American ring size 8. Dimensions: 0.6 x 0.6 x 0.2″ (1.5 x 1.5 x 0.4cm). Weight – 0.1oz (3 grams). Origin: Russia, Transbaikalia.
Some information about chiastolite:
Chiastolite is a variety of the mineral andalusite with the chemical composition Al2SiO5. It is noted for distinctive cross-shaped black inclusions of graphite.
There are several theories on the formation of the chiastolite cross, however the most accepted theory, proposed by Frondel in 1934, suggests that there is a selective attachment of impurities in the rapidly growing corners of andalusite crystals. As the concentration of these impurities (which consist primarily of graphite) increases, the growth of the crystal is slowed. This concentrated impurity deposit forms a re-entrant as it is absorbed by the growth of the andalusite porphyroblast. The cycle of growth-retardation-growth, then repeats itself, creating a featherlike pattern of graphite along four radiating ‘arms’.