People magazine reports that the ring appears to be a large, round bezel-set diamond in either a white gold or platinum setting, and is Art Deco-styled.  The magazine says that when Katy and John left the Bootsy Bellows club in West Hollywood she wasnt too worried about covering her hand up in public.

The magazine also says that this is the second Valentine's Day in a row that Katy has been seen wearing a ring.  Last year, it was a gold-and-ruby heart-shaped one, which turned out to be a gift from John, though not an engagement ring.

A source tells E! that despite the ring sighting, the couple is not engaged....yet.