Geodes
Hi, Kamron–
Since lava rock is composed of many minerals, the odds are that some of them
will dissolve away faster than the quartz of a geode embedded in the lava will.
Once some of the tiny grains are gone, the structural integrity of the lava
rock is gone, and the "tougher" minerals can be rolled away by wind or flowing
water without any dissolution or other reaction being needed. This breach of
integrity will not happen to the geode; a bit of the outermost quartz may
dissolve, but the inner layers will continue to hold the geode together. The
commonness of quartz in sand suggests that a great many minerals will weather
faster than quartz.
This is my own guesswork; I am not clear on which of the two factors
(structural integrity and rate of chemical weathering) is most important, and a
third may have escaped me.
–Doug