I've seen a lot of words use an X but be pronounced with a Z. Mitch Hedberg put it best:
Xylophone is spelled with an X, that's wrong, xylophone's zzzz, X? I don't $%(@#& see it. It should be a Z up front, next time you have to spell xylophone, use a Z. When someone says, "Hey that's wrong," say, "No it ain't. If you think that's wrong, you need to get your head Z-rayed." It's like X wasn't given enough to do, so they had to promise it more. Okay, you don't start a lot of words, but we'll give you a co-starring role in tic-tac-toe. And you will be associated with hugs and kisses. And you will mark the spot. And you will make writing Christmas easier. And incidentally, you will start xylophone. Are you happy, you $%(@#&' X!?!
So why is X used like this and where did it start?