On the outside, geodes appear to be nothing but rock, but looking deeper into the interior will reveal amazing growths of crystal formations. Natural gemstone geode crystals are usually composed of quartz or chalcedonic deposits, but various other minerals such as calcite, celestite and dolomite are also commonly found within other varieties of geodes.
Geodes are rock cavities or vugs with internal crystal formations ranging in a variety of interesting colors, including pale lilac to deep reddish purple, pale yellow to smoky yellowish brown, and ranging from transparent to translucent crystals. Agate geodes are often banded exhibiting very interesting patterns. Amethyst and citrine geodes can be small nodules cut in half or pieces of larger geodes sold as crystal clusters. Value per carat in geodes, unlike many gems, usually doesn't rise exponentially with weight as it is readily available in large sizes; but rather, the price depends almost entirely on the crystal growth and of course, color.
Crystal production that develops within a geode depends on several variables including the amount of moisture trapped within the geode, various chemicals and the variety of rich minerals deposited, as well as the amount of pressure applied to the geode. The entire process of geode formation takes millions of years, and not surprisingly, the majority of geode specimens are never completely filled out. Geodes with filled out crystal formations are often referred to as nodules; geode nodules composed of agate are referred to as 'thundereggs'.
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