This guide explains how to earn the My treasure achievement in Mountain level in detail.
Guide to Obtaining My Treasure Achievement
How to Do
Start the level Mountain and do what you usually do until you get to the train.
Go to the right and jump on the rocks until you see a cave.
If you go to the left, the wall is blocked. If you go to the right, it will be very dark. Bring the lantern and go to the right. Then, go to the right. There's one of the gems. Take it and put it below the hole. There are seven gems. That's one out of seven!
Hold the lantern with one hand and the gem with the other one. Then, go up the hill. Go to the left. Then go left again. If you go all the way to the left, you'll see nothing. Go to the "room" to the right and get gem #2. Go down and put it in place. 2/7!
One more to the right to get the third one.
One more to the right. Then the left. Get the fourth one. Then, go to one more to the right and drop down the hole. I just noticed my backpack has a gem... Then you will drop down to the wall where you got the lantern from. Go to the right and drop down the hole. Go down the hole and then put the fourth gem in a pile with the other ones. 4/7!
The 5th one... Go up the hill, go all the way up and go left. Get gem #5.
Gem 6 is on the right. The last one, up the hill, go to the left, go in the second room, and go to the right. There's another way to leave.
After passing the rocky wall, go left instead of right. Let go of the lantern and go up. Pry the bars with the stick and finish the level.