_Crystalline System_: None. _Form_: Amorphous. _Hardness_: 5.5-6.5. _Lustre_: Waxy to subvitreous. _Refraction_: Single. _Chemical Composition_: Silica 91.32. Water 8.68. _Chemical Symbol_: SiO_{2}, n=SiO_{2}nH_{2}O. _Specific Gravity_: 1.9-2.3. _Properties_: No electrical properties. Opal exhibits characteristic colour reflections known as Opalescence. It is susceptible to heat and weather changes, exhibiting greater brilliancy on hot than on cold days. The opal has never yet been successfully imitated, and certain peculiar properties yet remain to be investigated. Professor Frank Rutley F. G. S., emphasises the fact that "the nature of the Silica (Hydrous Silica) is not yet definitely determined." It is infusible before the blowpipe, but turns opaque. _In the Zodiac_: The Opal is under the Heavenly Leo, Libra and Aquarius. Leo favours red and fire opal; Libra, light translucent, pure colours, etc.; Aquarius, dark, black Opal, etc. _Where found_: Hungary, Honduras, Mexico, United States. The finest opal is now found in Australia, principally at Lightning Ridge, White Cliffs, Stuart’s Range, Charleville, etc. (Mr. Conrad H. Sayce gives (“Australasian,” March issue, 1920) an analysis of Stuart’s Range opal, bearing earth which contains about 35 per cent each of Alumina and Sulphur trioxide. He opines that this may account for the harmful effect it has on the men’s eyes and lungs.) PEARL _Hardness_: 3.5-4. _Lustre_: Translucent. _Chemical Composition_: Carbonate of Lime and Organic matter. _Specific Gravity_: 2.65-2.89. _Properties_: Affected by acids. Benefitted by some skins, adversely affected by others. Destroyed by fire. _Where found_: Persian Gulf, Ceylon, Red Sea, South America, New Guinea, Thursday Island, Australia, etc. _In the Zodiac_: Pearls are under the Heavenly Cancer. PERIDOT CHRYSOLITE—OLIVINE _Crystalline System_: Orthorhombic. _Hardness_: 6-7. _Lustre_: Vitreous. Transparent to translucent. _Refraction_: Double. _Chemical Composition_: Silicate of Magnesium and Iron. _Chemical Symbol_: (Mg,Fe)_{2}SiO_{4}. _Specific Gravity_: 3.3-3.5. _Dichroism_: Distinct. _Properties_: Friction induces electricity in the stone which is infusible before the blowpipe, but is affected, whitening and forming with Borax, a yellow bead. Decomposes in Hydrochloric acid. _In the Zodiac_: The Peridot is under the Heavenly Pisces. _Where found_: United States, Ireland, Australia, (Queensland particularly), etc. A large number of Meteorites contain Peridots. QUARTZ ROCK CRYSTAL, AMETHYST, CAIRNGORM, CHRYSOPRASE, CATSEYE, PLASMA, JASPER, CARNELIAN, AGATE, ONYX, SARDONYX, MOSS AGATE