Are you referring to the initial effect or the actual status affliction?
I haven't investigated this in depth, but it appears that the former is one of the many hardcoded 3D effects, in this case involving some light brown-ish 2D planes being animated to generate what is displayed when someone casts Stone Curse on a character. The latter isn't an actual effect, as the sprite is simply coloured differently while afflicted by the status effect.
There's some brown textures called "stone.bmp", "1", "2", "3", and "stone64", which you could try replacing, but I don't know if these are actually used for the effect. It's pretty hard to tell and I don't have time to do a frame-by-frame analysis to give a better answer, nor to reverse-engineer the client.