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