X is for XENOLITH. The only subject that I can find on my blog for this letter. This is a natural formation in granite rocks where a different kind of rock has been incorporated in the matrix.
https://natureinfocus.blog
Jessica's Nature Blog 
I have always found this interesting since it makes me imagine the formation of the partnership when the rocks were young and in motion.
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Yes, it’s a very dynamic process that took place deep in the bowels of the earth as liquid rock penetrated a chamber, and broke pieces off the lining of that chamber, which then became incorporated in the molten lava.
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