I've recently received a string of pearls that I believe to be Tahitian, they are a little bit gritty. But need some more information. I was told this forum could help.
I will answer any questions to the best of my ability.
To me they look like Freshwater pearls treated in some way to immitate Tahitian pearls. They appear to be a very dark black unless that is just the photos. Little real life to them but as long as you didn't pay too much they would be okay to keep. The price should have been well under $100 and preferably only $40 or $50 or less unless the clasp is gold. Where did you buy them from. China?
They're not Tahitian pearls, they look like dyed freshwater pearls. Freshwater pearls are often dyed to imitate Tahitian pearls, but your pearls are the wrong color. You can check some threads here like this one https://www.pearl-guide.com/forum/showthread.php?t=7352 to see how real Tahitian pearl color is.