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An advanced form of precipitation of minerals inside an encased stone. It is considered as a "solid geode". The minerals are commonly agate, chalcedony (a variation of silica) or calcite. It has an igneous origin.
For igneous origin, it is most likely due to the solidification high viscosity magma from perlite flow ( a volcanic rocks that has 2-5% of water content, Obsidian being less than 2%). The encased magma mix with earth mud and expand as a result of chemical reaction. With space present, minerals grow and precipitate in fully within. Some suggest that when perlite flow is cooled enough to a certain temperature, mud magma layer may shrink allow space for mineral precipitation.
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