The article asks about the etymology of X for crystal. I looked into that a while ago. The abbreviation "xtal" has been used for "crystal" since the 1800s in medicine, geology, and chemistry, and then electronics copied the usage. This comes from the earlier use of X for the "christ" sound, as in "xmas", which goes back to the 16th century. As the article suggests, the Greek chi (Χ) is the root.