All the knives that I came across and had cast aluminum handles,
he were very slippery, and during work left on the hands
of black traces of aluminum. I wonder if Ruana knives
are as slippery as that?

I think a good option would be to the handle with anodizing,
like on tactical aluminum flashlights
or microtech knives with anodizing handle,
this improves grip on hands and eliminates traces.