Dwarves' Earth Treasures:
Formation of Agates and "Thundereggs"
What type of quartz to be deposited in a cavity?

    What type of quartz to be deposited in the cavities depends on how much water is available (lakes, rivers, groundwater, rain, etc..) and how much silica dissolved in the water that are diffusing through the fractures in lava rocks. There are several forms of quartz that can be deposited from the solutions:

-Coarse crystalline form of quartz like those lining the geodes.
-Agate/chalcedony are the radiating fiberlike forms of microcrystalline quartz. "Agate" tend to be referred to banded/layered variety while "chalcedony" tend to be referred to bubbly/rose/irregular forms. "Blue Chalcedony" and "Purple Chalcedony" are actually agates even the banding are not quite visible.
-Opal consists of tiny quartz spheroids arranged in layers. If the sizes of spheroids are uniform, of proper size, and well arranged, the "play of fire" will occur as a result of light being diffused and refracted by the water filled gaps between the spheroids.

    It's very difficult to dissolve quartz in water, so where did the solutions get silica from? One idea was that they got silica from the feldspars, because the feldspars tend to be easily altered and decomposed into clay by water. Thus, water obtained silica in form of silicic acid from the feldspars by decomposing them into kaolinite (clay).

    If water becomes oversaturated with silica acid, opal and "waterline" agates will form, because the molecules don't have enough room/time to arrange themselves into crystals. The "waterlines" will shrink over time when drying, sometimes forming "mud cracks/peelings" in the bands.

    If water becomes saturated, chalcedony/agate will form. The molecules have slightly more room to move around a bit, but not enough time/space to form coarse crystals. Chalcedony are formed from rapid deposition of silica resulting from hydrothermal (hot) solutions entering into cooler water-filled cavities.

    If water becomes undersaturated, quartz crystals will result, because the molecules have room to arrange themselves into crystals before water dries up. The solutions sometimes degrade from saturated (forming agates) to undersaturated (forming a druzy coat of quartz on agate) if given time.

    Beside silica, the solutions often carry mineral impurities especially iron and other metals that act like coloring agents (dyes). Iron is one of most common metals on the Earth, hence, the colors of most minerals especially agates are provided by iron impurities. More concentrated the impurities in a mineral, stronger the color will become, but that is not always true. Excessive impurities would make a mineral look very dark or "dirty". Varying light and temperatures can affect the colors of the minerals. Bake a poor quality amethyst geode, and you'll get a bright yellow citrine geode. We had cases of blue fluorite from Bingham, New Mexico losing its blue color when exposed to sunlight too much.

    Any agates formed in any rocks high in iron like basalt tend to be more colorful while those formed in any rocks low in iron like rhyolite tend to be less colorful. The amethyst geodes are found in basalt rocks, and they got their purple color from iron impurities affected by natural radiation. But the agates in rhyolite can be quite colorful if any rocks high in iron like basalt happen to be overlaying or close enough.  We can even find the colorless agates in basalt rocks which may be due to not having enough iron impurities within the basalt rocks or they already have been leeched from the basalt rocks before the agates were formed.
    Iron impurities usually provides pink, red, orange, yellow and purple, nickel providing rare green color, and manganese providing black colors. The origins of deep blue colors are not quite known, but light blue color may be due to optical illusion (you has been tricked into seeing blue color).