There have been cases of fake Conch and Melo Melo pearls. But the pattern looks like the real Melos I've seen up close. If it's genuine, it's sure to be of great value to a collector. The best thing to do would be to send it to a gem lab that would be trusted by the purchaser.
Nice color, and heavens, is that BIG, or what???
Good Luck,
Blaire