Open spaces in rocks present places for crystals to grow unimpeded and thus host most of the well-formed crystals desired by mineral collectors. There are many different mechanisms by which open spaces in rocks can form. The resulting spaces and mineral deposits that precipitate within them are referred to by different terms. Herein we review some of those terms with a specific focus on geode, the topic of another article in this issue, Midwest geodes (Huizing 2017, this issue).
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