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