In its pure form, platinum is not magnetic. According to the University of California at Berkeley, platinum alloys can be magnetic. Precious metals such as platinum are too soft to be formed into hard objects, so they have to be mixed with small quantities of other metals when being used for applications such as jewelry making.
Because platinum has to be mixed with other metals, a magnetic platinum ring is not necessarily a sign of fraud. However, those who are concerned about the quality of their supposedly platinum rings that are strongly attracted to magnets may want to have the piece inspected by a trustworthy and certified appraiser.