I think the alum block is very similar to the deodorant crystal. In any case, it is very smooth & doesn't scratch the scalp at all. The only issue I have found is it is very brittle & cracks/shatters if dropped in sink or on bench.
There's a lot of talk of aluminium causing Alzheimer's but a google search said aluminium is fairly safe &, moreover, alum block is not really composed of aluminium. So I think it's all good.