Dwarves' Earth Treasures:
Formation of Agates and Thundereggs

    The flow and vicosity (how thick) of the lava, and amount of dissolved gases govern how the gas bubbles are shaped. If the flow is quick, the bubbles would be more stretched. But if the lava is very fluid and slow-moving, the gas bubbles would have time to try to escape upward or keep enlargening by absorbing other gases as in case of large famous Brazillian Amethyst geodes which tend to look pointy or look like cathradels. Everything always expand when heated and shrink when cooling and that is true for the lava rocks. As the lava cools into basalt, the fractures will develop and grow to connect to any cavities within the basalt rocks.

1. Picture of quartz-lined gas bubbles in basalt.
2. Picture of Brazilian Agate Nodules: Note how their shapes earned the name "Amybdaloid", a Greek word for "almond-shaped".
3. Picture of cathradel-like Amethyst Geode: Basaltic lava flows may had been very fluid with lot of gases dissolved in it. There may be some other factors for such large size of the amethyst geodes.