phaolo: Eh, it's literally called "Secret Achievement" and it doesn't have an icon, at least on my list (you can find it in Galaxy, in the "my Progress" tab)
Sorry but this wiil have to be a Loonng one;
It should be the secret area then, named:
Ancestry-Find the secret sector with the rock ship,
You will have to grind a good bit, while using a rock ship,
a event will pop up with a crio chamber in space,
pick it up, get to a zoltan sector, find a 2sd event,
this one a scientist will ofter to defreeze the guy,
its a crystal, that should lead you to a rock sector,
with a new event to the crystal secret sector.
Honestly its a nigthmare to get it, is my only one left.
There use to be a cheat code arround it,
where you would name a crystal 1 specific name
at the start, and so cutting in half the task,
but they patch it out i guess.
Good travels and all the luck to get this one.