My first guess is a gymnocalycium - have you seen it flower?

Thanks Jade, no, not jet:
davide

I agree, it looks like a Gymno, I'm thinking a little bit like a G. schickendantzii, slightly misshapen.
Dan
Lincoln, NE USDA 5

Looks alot like a Neoporteria jussieui to me.

Denise in PA

Neoporteria was the first name that came into my head, but they are so variable I wouldn't go further. I don't see any chins.

Yes it could be Neoporteria group but another that comes to mind is Acanthocalycium glaucum

Here is a link that might be useful: {{gwi:461849}}

Check this link to Denmoza, the fourth image
Alfonso

Here is a link that might be useful: Denmoza

I'm placing my money on Neoporteria, possibly paucicostata, (Eriosyce taltalensis ssp. paucicostata)or even jussieui. It's definitely not a denmoza or gymno. These plants take on this beautiful purple colour with strong sunlight, and green up with more shade.

Cheers CP