Thanks everybody for the comments.
Frank, this is just the skeleton of a quite common mediterranean sea urchin...and it is really violet/pink. Sometimes the most beautiful things in nature are just hidden "in the back yard"!
Rosko, I focused on the target (closer part of the subject), closed the diaphragm to f11 and the just moved the whole bellows with its own rail. I took around 40 shots. Of course I used mirror lock up and cable release.
BW, it was my first stacking, and it took me roughly 40 minutes from zero to the stacked photo....and I enjoyed every minute!!
By the way the RR is around 1,7:1