Re: Why do geodes have crystals on the inside and not on the outside?

The crystals you see inside of geodes grow when mineral-rich water passes through rocks. Water is present everywhere in the crust of the Earth and as water flows through pore spaces and fractures in rocks, it dissolves minerals in some areas and precipitates minerals in other areas (depending on a lot of factors like the temperature of the water, how acidic the water is, what other chemical elements are present in the water, etc.). You can study how crystals form from mineral-rich water by growing your own crystals at home. The most common mineral found in geodes is quartz.

In addition to the right combination of water and dissolved minerals, you also need cavities within the rock to form geodes. Some geodes form in volcanic lavas which have holes inside them which gas bubbles formed while the lava was still molten (the geodes from Dugway, Utah formed in this way). Other geodes form in fractures and voids (perhaps left by fossils which have dissolved away) in sedimentary rocks like limestones and shales (the geodes found in your home state of Indiana formed this way).

When silica-rich water first flows through a void in a rock it will often leave a rind of quartz coating the inside of the cavity. On this rind, crystals of quartz will begin to grow inward. In some geodes, the crystals completely fill the hole, in others they do not. When the surrounding rock weathers away, the round geodes are often left behind because the quartz rind is more resistant to erosion. When the geodes are cracked open, you find the crystals inside.