Are these sea water pearls

Hiya guys,I purchased the about pearl necklace recently.Thr pearls are 5 mms in diameter and the necklace is 39cms long.It. has a London nine carat gold hallmarks and weighs 62 grams.There is I believe some grittyness on the teeth,but I am a novice at pearl buying.Any help with identification would be greatly appreciated.Many thanks Gus

Whatever they are, they appear to need a good cleaning.

They are imitation pearls.