tortured soul stones

I am not really happy with the result, i tried to paint a screaming face, but the soul stones is just to small to do it properly.

So i wanted to hear if anyone had any alternative ideas as to how such a stone might be portrayed?

What the background for it could be would be something like this; Since The Dark Eldar doesn´t have any bonesingers like their craftworld cousins (since all psychic powers are banned from the Dark City)
they would use an imitation of the souls stones.
Yet this soul stone unlike that of the Eldar isn´t used to capture a soul, but rather all the pain and torment of their captives.
They use this energy from the stones to force, rather the guide as the bonesingers do, the wraithbone into the shape they want it to be.

I am thinking to take a hint from Matt Smiths first into in Doctor who this maelstrom with perhaps a single lightning bolt in it, but i am not sure that would get the idea across.
So how would you portray tormented soul stones?

My approach on (broken) Soulstones.

Green stuff, not painted yet.
They could be just poison splinters though.
I plan to paint them like the agonizer on this, maybe paint 2D flames around them (see sig.):

I don't think i could paint tortured souls in them either.