. Pierres précieuses, un compte rendu populaire de leurs personnages, l'occurrence et d'applications, avec une introduction à leur détermination, pour les minéralogistes, lapidaires, bijoutiers, etc. avec un appendice sur les perles et le corail. Pierres précieuses ; Perles ; les coraux. 295 spinelle spinelle. Les pierres qui ne sont plus pris en considération de manière adéquate après corindon sont ceux appartenant aux groupe spinelle de minéraux. Leur couleur ressemble souvent à celle de la ruby, mais dans tous les autres caractères les deux minéraux sont parfaitement distincte, de sorte que les noms " spinelle rubis" et " balas-ruby,'" qui a des

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. Precious stones, a popular account of their characters, occurrence and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. With an appendix on pearls and coral. Precious stones; Pearls; Corals. SPINEL 295 SPINEL. The precious stones which are most appropriately considered after corundum are those belonging to the spinel group of minerals. Their colour often resembles that of the ruby, but in all other characters the two minerals are perfectly distinct, so that the names " ruby-spinel" and " balas-ruby, '" which are sometimes given to certain colour-varieties of spinel, are misleading and incorrect. Spinels of a sapphire-blue colour are also known, but, like black spinels, they are of little importance. In scientific mineralogy the spinel group includes a very large number of minerals of varied composition but of identical crystalline form and chemical constitution. Of all these minerals of the spinel group, differing widely from each other in chemical composition, hardness, colour, transparency, &c., there is but one which is generally used as a gem, and this particular stone is therefore distinguished as precious spinel, or noble spinel. Precious spinel is a compound of alumina (the sole constituent of ruby) with magnesia, its composition being represented by the formula MgO.AljOg or MgAlgO^. This compound is in itself colourless, so that the various colour-varieties of spinel owe their tints to the presence of small quantities of foreign substances. The colour both of the ruby and of the red spinel is thought to be due to the presence of chromic oxide (CrgOg), but, according to Doelter, the presence of the small amount of iron, as shown in the following analysis, is sufficient to account for the colour of these stones. The analysis referred to was made by Abich on a red spinel from Ceylon. Alumina (AljOj) . . 7U'43 per cent. Chromic oxide (CrgOs) . . 1'12 , , Magnesia (MgO) . 26-75 „ F

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