Dwarves' Earth Treasures:
Formation of Agates and "Thundereggs"

Glossary:

-Silica: This terms applies to a mineral in form of quartz, agate, chalcedony, and opal. Each silica molecule consist of a silicon atom combined with two oxygen atoms in triangle-like arrangement. Put together six triangle-like molecules and you get a hexagonal shape, characteristic of quartz crystals.

-Cristobalite, a form of quartz that crystallizes only at very high temperature as in case of cooling lava, and in several ways, cristobalite is very different from regular quartz. Cristobalite is the main "ingredient" of the thundereggs.

-Basalt: A type of dark colored igneous rock that fluidly flowed out of dikes, fissures, and shallow volcanos associated with rifting and faulting. Such rocks are high in iron, and dark minerals but not high in silica (50% or lower). The volcanos of Hawaiian Islands, and Valley of Fire of New Mexico are the examples that produce this type of igenous rock.

-Rhyolite: A type of light igneous rock that is very thick due to its high silica (75% or higher) content but very low in iron. Rhyolite is associated with the volcanos along tectonically active mountain ranges. The volcanos in Washington, Oregon (St. Helen) and Northern California are examples that produce this type of rock and there are several varities of rhyolite.

-Obsidian and Perlite are both lava glassy rocks commonly associated with rhyolite.  The difference between them is that obsidian is hard and breaks like glass while perlite is glasslike but very brittle. Unlike obsidian, perlite is hydrated (containing some water), and that is why perlite easily decomposes into clay while American Natives find many uses for obsidian.  Lower the water content, harder the pertile would become.

-Amygdaloid (Greek name for "almond shaped") refers to any cavities within any solidified bodies of lava that are formed by the gas bubbles trapped in the lava. Such cavities are typical of basalt lava rocks, but they sometimes also occur in other lava rocks such as rhyolites. Typical minerals found in such cavities: Quartz, opal, zeolites (thomsonite, natrolite, etc) and their associates (apophyllite, datolite, etc.), calcite, native metals, prehnite, chlorides, and epidote.

-Lithophysae (Greek name for "Stone Bubbles") is a scientific name for "thundereggs" that refers to the bumpy sphere-like bodies in any silica-rich solidified bodies of lava especially rhyolite and perlite that had cavities opened within them by gases released from the lava. The sphere-like bodies lacking any cavities are Spherulites which we would consider duds. Typical minerals found in such cavities: Quartz, chalcedony, opal, calcite, manganese minerals, & clay minerals.

-Concretion refers to the nodules found in sedmentary rocks like limestone and shale. They appear to result from accumulation of minerals around an object like fossils or minerals that will eventually have its insides be dissolved to form cavities to be later filled in by the minerals. Utah Septarians are the famous examples of concretions. Typical minerals found in such cavities: Quartz, chalcedony, agate, calcite, aragonite, dolomite, barite, pyrite, millerite, gypsum, and clay minerals.

-Geode refers to ANY nodules (amygdaloid, lithophysae or concretions) that are hollow and usually lined with crystals. I would rather use the term "nodules" since I don't know what's inside a nodule until I break/cut it. If it's hollow, it's geode. I even had used "geodic thundereggs" to describe the thundereggs wtih hollow quartz-lined cavities.

-Open System: A term described by one theory refering to the idea of the agates formed from the solutions in successive layers thru the depostional channels in respect to the oscillating weathings (dry & wet periods)

-Closed System: A term described by one opposting theory refering to the idea of agates formed entirely from the solution isolated/sealed within a cavity and the crystallization would bulid up pressure that would somewhat deform the agate layers.

-Diffusion: The movement of dissolved minerals from higher concentrations to lower concentrations. For example, in case of a tank of of salty water connected to a tank of fresh water, dissolved salt will automatically move from a tank of salty water into a tank of fresh water until both tanks have equal amounts of salt.