"Die Ruhe kommt von allein."

[ArmasUkkoM]

"von allein" is a more or less fixed phrase meaning "without others' influence".

[Mifty]

Why naturally and not alone?

[philster043]

It sounds better in English, I guess. The calm wasn't caused by exterior causes, it just happened on its own - hence "naturally." "Alone" makes it sound a bit peculiar.

[Hans_Allein]

Peace comes from within or Peace comes from solitude would be better translations

[bf2010]

Calmness comes on its own

[Gustav_Lauben]

"Quiet comes from alone" does not sound to me like a real English sentence! I don't think I'd understand what someone meant.