distingushing feldspar n quartz by physical properties
Actually, quartz can sometimes have purple (or other colors, such as yellow) depending on the impurities present in the rock. Likely, it *is* quartz. However, if you post a picture and ask the question again, someone with more knowledge than I could possibly identify it.
It sound like you might have a piece of agate, though it is virtually impossible to ID a rock without a picture. You can do a Google search for rock ID and you can find some good sites.
