I have a Cross 4502. It is a 10kt rolled gold pen. It is already old but it was only given to me last 2 months ago. I have carried it seldom but it has always looked the same. Just moments a go, I grabbed it from my desk and noticed that it was coloring brown and about 70-80% of the pen is covered. There some small spots wherein the same gold color is seen but the brown marks are very noticeable(they are slightly hard to see in the picture because of the flash) What might have happened?