These relics have a weird "Y"-shape as you see in the achievement picture.They are directly visible and not inside a chest or any other sort of container.
As soon as you pick up your first one, you will get a quest that keeps track of how many of the 30 relics you have collected so far("Ancient Relics").
Note that there's a fourth kind of collectable, the shards of a runestone(quest:"The forgotten graves") but those aren't related to any achievement.
All collectables are missable which means you need to pick up everything in each area before moving on to the next.
For detailed locations see ponypo2001 's awesome collectables guide.