Without seeing the back, it is a bit of speculation but I would guess this is costume jewellery; the verdigris where the rhinestones are missing is evidence these were foil backed rhinestones - I would be guessing 1950's, 1960's but it could be as early as as 1930's.
Again a photo of the back would be helpful but with these stones it is unlikely to be any precious metal; I would be guessing some white metal, like nickle, plastic pearl beads, blue paint. If complete in good order it would be quite attractive (though not valuable); however these missing stones would be virtually impossible to replace, and its value I'm afraid is very little. Of course if this is a family piece it doesn't really matter.
