Aha! Then you put a tiny corundum crystal inside a tiny padlocked box, and permanently affix it to the ring. (Or perhaps the ring has a lockable box portion.)

Aha! Then you put a tiny corundum crystal inside a tiny padlocked box, and permanently affix it to the ring. (Or perhaps the ring has a lockable box portion.) Now the ring cannot be stolen without also stealing an object that is inside a padlocked box. Safe to mail.