Sounds like you have a Spaceview Accutron. Most all of those have a plastic crystal, not mineral glass. Does your Spaceview already have a repro crystal on it? If so, you could purchase another of the same type. Yes, there are several different designs particular to certain Spaceviews. Also, there are different diameters. It isn't possible to recommend one w/o more info. If the case code, stamped in ink inside the case back, is still there you may be able to determine the proper crystal from that code. Unless you have the tools, let a watchmaker, not a Jeweler who just does batteries and straps, do the replacement.