My argon gas-filled replacement windows are six months old. Two of them
have developed strange patterns of silvery purple flecks that appear to
be on the inside of the glass.

Take a look: http://www.704westjoppa.com/window.gif

Any ideas what this could be? Reaction of the gas to something inside
the window? Broken seal on the window?

My window guy is coming out next week, but he hasn't heard of this
problem either.

Other thoughts?