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First there are the irregular shaped kind [select geodes at this great site] that formed in odd shaped openings under ground.
Second are those that formed as silica chains or needles in silica melts at temperatures just above the melting point of 1,687 Degrees K. That is the kind we are talking about here.
Third are the kind that formed as silica melt spheroids [Brazilian Geodes etc.] at temperatures between 1,687 degrees K and 3173 degrees K.
