Heres an update:
Seeing that no damage had occurred over after a couple of weeks of spot cleaning with acetone.
I got all excited and cleaned a whole pawn and waited again for another week. no damage seem to be apparent and all the detail remained. Could this work I start thinking.
I had cleaned all the pieces to bring them to remove all the grime and the blue. The only problem is that it brought them all to a bright white as seen by the 4 on the left

I wanted to color the peices without harsh chemicals. Thinking about this over a cup of tea one day the idea just came to me ... TEA! I started to stain one pawn, by leaving it in a jar of tea for about a week. That seemed to come to a light brown color. I threw the rest in and waited, stired, waited, stired, and waited some more, Now I have a nice light brown color of pawns.

I think that it gives it a 'rustic' look to the pieces.
I'm now leaving these for week or so to make sure that everything goes well.
Will do another update when the are all done.
Hey all,
I got my self a second hand chess set with the Isle of Lewis pieces and even the stone board - YAY
However it looks like someone tried to color some of the pieces with a black pen and someone else has tried to clean it off.
I've tried Googling around on what to use to clean it with out losing the details an unable to find anything useful and I'm a bit wary about damaging the set.
I've tried water and a tooth brush, with no avil, I tried a small patch with Acetone, and that's cleaned it up really well.
It might have taken off a protective layer off the stone too,
Does anyone know/think that this might do long term damage to it and if there is anything that I can reapply to it to seal the stone again?
Thanks in advance