<p>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are about 5400 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
</p><p>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  "Silica" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.
</p><p>The simplest &amp; most abundant silicate mineral in the Earth's crust is quartz (SiO2).  All other silicates have silica + impurities.  Many silicates have a significant percentage of aluminum (the aluminosilicates).
</p><p>Quartz (silicon dioxide/silica - SiO2) is the most common mineral in the Earth's crust.  It is composed of the two most abundant elements in the crust - oxygen and silicon.  It has a glassy, nonmetallic luster, is commonly clearish to whitish to grayish in color, has a white streak, is quite hard (H≡7), forms hexagonal crystals, has no cleavage, and has conchoidal fracture.  Quartz can be any color: clear, white, gray, black, brown, pink, red, purple, blue, green, orange, etc.
</p><p>Shown above is the interior of a geode.  Geodes are small to large, subspherical to irregularly-shaped, crystal-lined cavities in rocks.  They form when water enters a void in a host rock and precipitates crystals.  The most common geode-lining mineral is quartz.
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<hr><p>Geode info. from the Field Museum of Natural History (Chicago, Illinois, USA):
</p><p>"Geodes are hollow, subspherical bodies, ranging from an inch or two to a foot or more in diameter.  Most geodes occur in limestones, rarely in shales.  They have an outer chalcedonic silica layer which is separated from the enclosing limestone matrix by a thin clay film.  The inner surface of the chalcedonic layer is usually lined with inward projecting quartz crystals, though in many geodes drusy coatings of calcite and dolomite occur commonly.  Of less common occurrence, are crystals of magnetite, pyrite, sphalerite, and a few other such minor and rarer constituents.
</p><p>The mode of origin of geodes in sedimentary rocks is but imperfectly understood.  That geodes originate in an initial cavity, such as the unfilled space within a fossil, is well recognized, but whether such a cavity is a necessary prerequisite is open to question; geodes may originate in cavities formed by solution.
</p><p>Many geodes show evidence of expansion, apparently resulting from pressure.  A notable example of this singular phenomenon of expansion of the growing geodes is the "exploding bomb" structure.
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<hr><p>Photo gallery of quartz:
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&lt;a href="<a rel="nofollow" class="external free" href="http://www.mindat.org/gallery.php?min=3337">http://www.mindat.org/gallery.php?min=3337</a>" rel="nofollow"&gt;www.mindat.org/gallery.php?min=3337&lt;/a&gt;
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/32612594515/">Quartz-lined geode (Morocco) 2</a>