Here’s a nice text guide for all of you that prefer these over watching a video, myself included.
On the map, “Mountain”, once you get to the first red train car, head right along the cliff wall until it goes left and to a small plateau. This is where the cave entrance is.
When you head inside, taking the first left will get you the lantern. You cannot go without this, as it is pitch black inside and you cannot see otherwise.
It’s easiest to grab the lantern with one hand, and use the other for the gems.
The gems are in no certain order, so I’ll just number each set of directions at each intersection, with them being left, right and straight directions from the entrance, then directions to get back as well.
i.e. the lamp is : Left. Then: Right, on the way back.
1- Right, Straight, Right. (Just hug the right wall).
Left, Straight, Left. (Just hug the left wall).
2- Right, Left (up the hill), Straight, Left.
Right, Left (down the hill), Right, Left.
3- Right, Left (up the hill), Straight, Right, Right.
Left, Straight, Left (down the hill), Right, Left.
4- Right, Left (up the hill), Left, Left, Left.
Right, Right, Right (down the hill), Right, Left.
5- Right, Left (up the hill), Left, Left, Right, Right.
Left, Left, Right, Right (down the hill), Right, Left.
6- Right, Left (up the hill), Left, Straight.
Straight, Right (down the hill), Right, Left.
7- Right, Left (up the hill), Left, Right, Left.
Right, Left, Right (down the hill) Right, Left.
With them all brought outside, just put them all in a nice pile around eachother, no need to be stacked, just close together.
And there’s you’re achievement!