In the sand next to where he'd put his towel, Joey noticed something which sparkled brightly. It was some kind of precious stone, he realised, and he picked it up, looking over it curiously. It was a deep purple, so he guessed it was probably an amethyst of some kind, though he was no geologist so he suspected he could be wrong. In any case, it was quite pretty, and he found himself staring into it in fascination.
After a while, the amethyst started to glow, and for a moment Joey's eyes went hazy. The glow vanished, and all of a sudden Joey felt weird all over. He dropped the amethyst onto his towel and looked down at himself, his skin seeming to almost sift around as he breathed heavily. After a few moments, it mostly stilled again, but everything felt like it had taken a different shape or size. His shoulders had narrowed and his hips had widened, his skin seemed softer, smoother and more supple with most of its little hair having receded, and his voice seemed higher pitched as he realised the bulge in his throat had vanished.
Having paused, the changes soon continued and felt stranger still. Joey gasped in his softer voice as his chest grew sensitive, and his nipples stiffened and swelled up. Before his eyes, mounds of flesh emerged around them that felt sensitive and jiggly. He tried to deny it, but he knew what these were. Breasts. One arm reached to cover them in almost instinctive embarrassment.
Just moments later the changes moved downwards, and a mix of fear and curiosity clutched at Joey's mind. Looking around and to his relief seeing no one around, he pulled his swim shorts down a little to see his erection, and watched as it slowly shrank away. It felt like everything extending from his body was getting sucked back inwards. His balls were pulled up inside and started to reshape, his penis's length steadily declined until only a tiny sensitive nub remained, and between his legs a void seemed to extend out until it split his scrotum into a vagina.
As the changes finished, Josie hastily covered herself and looked in shock at the amethyst. What was this thing?