Geodes typically have a chalcedony (cryptocrystalline quartz) shell that is lined internally by various minerals, often as crystals, including calcite, pyrite, kaolinite, sphalerite, millerite, barite, dolomite, limonite, smithsonite, and quartz, which is by far the most common and abundant mineral found in geodes. Geologists use the term "chalcedony" to describe both natural materials that are pure or nearly pure quartz and those that contain other minerals too.
The majority of geodes we find in nature are micro-crystalline chalcedonies with average grain sizes of about 1mm. Larger grains can be found in some rare cases such as in some large agates. Geodes may also be composed of amethyst, apatite, aragonite, carbonate rock, chrysoprase, jade, marble, opal, onyx, petrified wood, pumice, salt, selenite, soapstone, stromatolites, syenite, or titanite.
Micro-fissures within the geode allow water to enter and fill it with rain or meltwater. As this water evaporates, the minerals in the fissures absorb energy and slowly expand, causing the geode to crack open. This process continues until the weight of the geode becomes too great for its shell to support itself, at which point it collapses in on itself.
Clear quartz crystals are found in most geodes, whereas purple amethyst crystals are found in others. Others may have agate, chalcedony, or jasper banding, as well as crystals like as calcite, dolomite, celestite, and so on. In general, the higher-quality the geode, the more valuable it is; also, the larger the crystal, the more valuable it is.
Geodes form when volcanic ash falls into shallow groundwater that contains calcium carbonate. The ash provides the necessary chemicals for crystal growth. Over time, additional minerals can dissolve out of the ash and be incorporated into the growing crystal. As the mineral concentration increases, so does the density of the rock. Eventually, the weight of the rock above the water line forces all the water out, leaving an empty shell behind. This process can happen very quickly, sometimes in as little as a year, but it can take much longer if the conditions aren't right. A geode will often have different colors within its body, which come from different levels of mineralization. For example, white geodes usually contain more calcium carbonate than colored ones.
Geologists use the term "geode" to describe any hollow rock formed by this process. Although many people think of geodes as being single rocks with large holes inside them, this is not always the case.
Geodes are typically spherical, with some being egg-shaped. Geodes, when shattered or sliced open, expose an inner lining of crystals or other minerals. Many of these crystals, such as the purple quartz known as amethyst, may be extremely lovely. Some geodes even have liquid petroleum within them.
Amethysts are forms of quartz that contain much more mica than ordinary rock quartz. This makes them transparent and colorless, but also very brittle. The color of amethysts usually comes from tiny inclusions of other minerals: brown iron oxide, black manganese dioxide, white calcium carbonate, and yellow chrome oxide are examples. Although most amethysts are clear, some have small amounts of colored glass mixed in. These include red, green, blue, and violet shades of crystal.
Amethysts were originally used as jewelry pieces, but now they are popular as decorative objects in their own right.
There are several varieties of amethyst, depending on its origin. Deep purple amethyst is found in South Africa. Light to medium pink amethyst can be found in Canada and the United States. To make dark red amethyst, heat pale pink or white amethyst in a furnace. Or put it in a pot of hot water! Purple geode paint is made by mixing natural amethyst with oil-based paints. The result is a gem-quality material that can be cut into beads or worn as jewelry.
Amethyst crystals and black calcite are found in the rarest and most precious geodes. An amethyst geode from Romania is valued at $300,000(USD). Black calcite specimens can sell for up to $50,000(USD).
Geologists have discovered that mare basalts contain significant amounts of quartz, rhodonite, magnetite, and other valuable minerals. But because these basaltic rocks are very porous they usually contain little or no gem quality material inside them. It is only after they are intruded by silicate minerals such as pyroxene or olivine that they become valuable geodes.
Basalt is one of the three main rock types that make up a volcano (the others being lava and pumice). They are hard and durable and can be found everywhere where volcanoes exist.
In fact, all the major gemstones (including diamonds) originated as microfossils within volcanic rocks. As the mineral grains grew larger due to heat and pressure their color and luster changed and were visible even under natural light. This is why gem-quality materials can often be found in common, unpolished volcanic rocks.
Geodes may appear to be ordinary rocks on the exterior, but as you know, they contain gorgeous crystals on the inside, making them ideal for exhibition in a container. Geodes are highly naturally formed rocks, therefore avoid rocks that are sharp or narrow. Also, make sure the hole in your geode is not too large because this could allow moisture in.
There are several types of geodes including:
Agate - has red and white colors with a mixture of quartz and mica
Amazonite - dark green to black colored stone containing small opaque white crystals of calcium carbonate
Andesite - light to dark gray color with occasional brownish-red streaks caused by iron oxide particles
Aragonite - pale yellow to white color with a grainy texture composed of calcite (calcium carbonate)
Biotite - dark green to black colored stone containing small clear quartz crystals
Calcite - same color as biotite but with no quartz content; used for timekeeping due to its crystalline structure
Chlorite - blue to green color with a granular texture made up of magnesium and chlorine minerals