Here is the full list of all 14 Just Bones achievements.
Complete ten levels with three stars
Die in lava for the first time
Find twenty keys
Complete all tutorial levels
Find ten keys
Get three stars for the first time
Find your first key
Fall into pieces for the first time
Complete twenty levels with three stars
Jump into swamp for the first time
Complete cave section
Complete mountain section
Complete swamp section
Complete thirty levels with three stars