Cache clous were designed to hide the picture nail in a most delightful way! Beautiful rose design to add that perfect French finishing touch to your boudoir or salon. 
So many other uses too, I have a couple in a little vignette box as I adore the deep lustrous patina ormolu acquires.

These are getting much more difficult to find, I have sourced a few for your delight! 
These delights are approximately 2.5cm or 1" in diameter. 

Ormolu antique cache clou