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PRECIOUS STONES.
97
but is not, as a rule, produced to any marked degree by the direct act of light, especially in the case of larger stones, though smaller ones may more often show it; when, however, the mineral is rubbed on cloth or paper, most stones are found to become phosphorescent, though the light emitted is as a rule not strong, and does not last for any considerable time ; yellow is the commonest colour shown. The high tension electric discharge from a vacuum tube causes Diamond to phosphoresce.
Under X-rays, Diamond is found to be remarkably trans­parent, a Sapphire appearing quite opaque in comparison; thus a convenient means is at hand for examining a large parcel of stones, other substances which could be substituted for Diamond casting a shadow, and thus discovering any attempt at substitution.
Under the influence of heat no change is observed as a rule until high temperatures are reached, though a few specimens may phosphoresce on heating, but all have this property destroyed by great heat. Moissan gives the temperature of ignition in oxygen as 690° to 840° C, but if heated out of contact with air or oxygen no change occurs up to an intense white heat, and then only a very slow conver­sion into the allotropic form of carbon, Graphite, takes place (vide infra). Diamond is a good conductorof heat; hence mois­ture rapidly condenses on it and rapidly evaporates again.
With regard to its electrical behaviour, Diamond shows a charge of positive electricity when rubbed, and this whether the faces are in the natural form or cut; it is of a very high electrical resistance, a fact of greater interest since other forms of carbon, as Graphite and coke, are good conductors.
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