Tobias Wong's "Killer Engagement Ring" is a sturdy diamond ring whose point faces out, turning it into a potentially lethal weapon.
The diamond sharp edge will also cut skin down to the bone (with a minimum 1 karat stone – but the larger the better). Or it may simply be used to tag hard surfaces, like cars and windows for S.O.S. messages or that last will and estimate when pen or paper (or lawyers) aren't conveniently around.
(via Making Light)