Agate Small Geode cut into two halves 45.3 gr in total mm 42 x 36 x 27 Raw Minerals Stones Rocks for Collection, polished edge, cave with crystals (Hyaline Quartz, Pink Quartz, Amethyst, etc.), only a piece, as in photos.
Also available in lots, at this link.
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Geode is a secondary geological formation within sedimentary and volcanic rocks. Geodes are hollow, loosely spherical rocks in which masses of minerals, which may include crystals, are isolated. Geodes are formed from masses of magma whose non-homogeneous and very slow cooling inside allows the minerals to arrange themselves in an orderly manner according to the crystalline lattice. In fact, they represent gas bubbles variously modeled by the greater or lesser flow of the magmatic liquid. The lower the viscosity of the liquid, the more elongated can be the shape of the geode whose sharpest part coincides with the direction of flow.
Groups of "grown" crystals can develop on the internal walls of these geodes thanks to the particular chemical composition and the relatively high temperature of the hydrothermal fluids percolated through the rock. Most geodes contain clear quartz crystals, while others have purple amethyst crystals. Still others may have bands of agate, chalcedony or jasper or crystals such as calcite, dolomite, celestite, zeolites, selenite, carbonates and sulphates, etc. The banding and coloration of the geode are the result of varying impurities.
It can vary in size from a few centimeters up to, in exceptional cases, a few metres, real caves.
Geodes come from Brazil, Uruguay, Namibia, Mexico, United States. Geodes are also abundant in the Mendip Hills in Somerset, England, where they are known locally as "potato stones".
Quartz (silicon dioxide, SiO2, is the second most abundant mineral in the earth's crust (about 12% of its volume) after feldspars.
The etymology of the name is not certain, but for the ancient Greeks quartz was defined as crystal (from the Greek κρύσταλλος, krýstallos, ice). In fact, they considered this mineral a variety of ice cold to the point that it could no longer be dissolved.
Quartz has a crystalline structure made up of silicon and oxygen tetrahedrons joined together by the 4 vertices to form spirals. The habitus (the shape) is a hexagonal prism with the faces of two rhombohedrons at the vertices arranged in such a way as to form a hexagonal bipyramid. Quartz can have an elongated prismatic shape.
Quartz is a common constituent of acidic intrusive magmatic rocks, among which the best known is granite; it is also abundant as a component of sedimentary rocks, preferably in sandstones due to its high resistance to chemical degradation by atmospheric agents and its insolubility in water and in metamorphic rocks.
Being one of the most common minerals of the earth's crust, quartz has a large number of different varieties, with macroscopic crystals visible to the naked eye, or crystals so small as to be visible only under a microscope or even invisible with such an instrument. Although many of the variety names have historically come from the colors of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Usually, but not always, color is a secondary identifier.
In addition to hyaline quartz, perfectly colorless and clear, there are numerous other varieties of quartz, many of which are for sale on our site:
Chalcedony, Agate, Moss Agate, Chrysoprase, Onyx, Carnelian, Sardonyx, Jasper, Heliotrope, Aventurine, Tiger's Eye, Cat's Eye, Bull's Eye, Hawk's Eye, Amethyst, Ametrine, Citrine or Madeira, Rose Quartz, Quartz blue, Prasiolite, Milky Quartz, Smoky Quartz, Morion, Rutilated Quartz, Tourmaline Quartz, Dendritic Quartz, etc.
Quartz is a material with remarkable chemical stability and is resistant to acids except hydrofluoric acid. It has high hardness (7 on the Mohs scale), mechanical and heat resistance. The quartz has no cleavage. Some physical properties of quartz crystals are piezoelectricity and pyroelectricity, i.e. the ability to electrically polarize the opposite faces of the crystal following the mechanical deformation caused by compression or after heating. From an optical point of view, quartz has high transmissibility in the visible and especially in the ultraviolet.
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