The stone fused with Clara's forehead and started to transform her. First, fine scales the color of the stone appeared all over her boddy, claws poked out from her hands and webbing developed fast, to help with swimming. Muscles started to bulge and fins ripped out of most of her joints, side of the arms and her back, all spiked with poisonous and venomous barbs, her breasts lost their nipples but were kept, albeit smaller on her muscular frame. Her legs fused in a eel like tail that was covered in plenty of the same fins she sported on her back and arms. Lastly, her head started to elongate, and turn into a downwards snout filled with two rows of saber like tooth, the hair becoming a huge fin that forms a hood and that can be used as a frill when in danger, the eyes became those of a mindles predatory beast from the deeps, ears turning into gill fins, gills that are sprouting all over her underside and sides of the boddy. But the stone wasn't done yet, it started morphing her more, doubling her size and making her more muscular and monstrous, it started breaking her mind, no longer believing that she was a human, or even female for that matter. It made her think she was a teen siren, looking to start a colony and that she was a hermaphrodite sporting both genders in the cloaca. Mental changes asides, the gem started forming markings on it's boddy making it look fairly atractive to it's main prey/reproductive vector, thus along the place it stood on the forehead, it made the markings resemble a human female face, while those on the body were a mix of camouflage and enhancing patterns that the human mind will mistake as a female swimmer. Still, this is not everything the stone did, as for the last two major changes it, firstly cover the whole body in a gelatinous slime form of the venom the barbs of every fin can inject and made most of the scales used to make the female mimicking markings be harder and made from the same stuf as itself.
After all this was done Clara was no more but the creature she became found it's first school mate.