( SiO2 ) © 2004 gggems.com All rights reserved. (Variety of Quartz) · Crystallographic properties: Trigonal System (Subdivision of the hexagonal system) Quartz forms hexagonal prisms at the blunted ends and the head finishing in hexagonal pyramids. · Physical properties Hardness 7 Density 2.65 - 2.66 Refractive Index: 1.54 -1.55 + 0.009 positive uniaxial Glare: vitreous · Chemical Composition: Faceted Amethyst Silicium Dioxide ( SiO2 ) The colouring of amethysts is due to the presence of colour centers which come from substitution of ions of silicon by iron ions in the crystal lattice of quartz. The amethyst crystallizes at temperatures lower than smoky quartz for example. In Madagascar, we find it either in the crypts of pegmatites, or in the quartzite veins in connection with those. The geodes of the siliceous nodules of basalts contain some too. The first have an hexagonal network, although their pattern of crystallization is only of ternary order. The seconds have a ternary network. The elementary mesh is a rhomboedron, i.e. a parallelepiped consisted six equal rhombuses. A ternary axis A3 joint the tops of the regular trihedrons, three normal binary axes A2 with the ternary axis joining the meddle of opposite horizontal corners. Here elements of symmetry of the Trigonal system with the oblique shape, the tetragonal scalenoedron. It is interesting to see what becomes this tetragonal scalenoedron in others classes where the disappearance of the symmetry planes makes decrease of half the number of the faces. We obtain a trapezohedron (One notices that these two trapezohedrons are not superposable. It is said that they are two enantiomorphism shapes. They are symmetrical compared to a symmetry plane.) One speaks about right trapezohedron and left trapezohedron (just as we have a right hand and a left hand, nonsuperposable thus enantiomorphism). Certain minerals present sometimes left forms sometimes right forms. The most known example is that of quartz. The distinction between left quartz and right quartz has a certain importance in the electronics industry where one uses the piezo-electric properties of this mineral. The negative crystals of hexagonal shape can contain liquids like water, gas, as well as rutile needles, tiny hematite discs, goetite, fushsite or mica. Inclusions of H2O and CO2 in an amethyst from Madagascar. Habitus type of flattened Dauphiné. This marvellous Bi-terminated amethyst of 128 grams also contains water drops in its center. Habitus type of rhomboedron. Enlarge The amethyst scepters are also typical in Madagascar. Here, a scepter in the blackberry shape. There, the stem of the scepter developed by crystallizing around it a quantity of small points in hedgehog. Length:40 mm. Double scepter. Enlarge Head of scepter: 132 grams Enlarge Amethyst in castle on its quartz gangue. 986 grams. Enlarge The fact that sometimes only a thin surface layer of violet color be presents in the stone or that this color is not homogeneous, makes often a difficult cutting. The art of lapidary is to place the color correctly in order to make homogeneous the tone of the cut stone. The distribution of the color in a cut stone is observed simply by immersing it in water contained in a transparent container and colourless. The color of amethyst varies from purple reddish to purple bluish while passing by straightforwardly crimsons tone. Thus of very variable intensity, the color is often laid out in stripes parallel to the final faces of the crystal.

Enlarge ¦ Cutting Styles ¦ Characteristics ¦ Crystalline Systems ¦ ¦ Amethyst Crystal 1 ¦ Amethyst Crystal 2 ¦ Amethyst Scepter ¦ Faceted Amethyst ¦

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