eBay
Here is a pair of hand-made Green Cobalto gems for Campagnolo C-Record Coblalto Brake Calipers (Just the gems are for sale, NOT the nuts.  But I have included two pics of the gems in the nuts so you can see what they look like).
 I made these out of frustration in trying to find quality replacements for a set of brakes that I have that are missing the gem stones.  All I could find on ebay was some that came from craft and hobby stores OR some that sellers had taken from actual cobalto brakesets and were selling for way too much.  So, I decided to create my own.  After many prototypes using different techniques and materials I finally came up with a pair that I would say is an excellent replacement for the original.
  Again, these are NOT from a hobby store.  These are actually hand-made (by me) using skills I have learned making hand-crafted fly rods (I know... an odd blend of the cycling world and the fly fishing world).
Each of these actually fit your cobalto brake center nut perfectly, including the way that the edge of the "gem" covers the perimeter edge of the nut.  All you need to do is do a proper glue job to hold the "gem" securely in the nut.  (I suggest using an epoxy made from two parts).
I plan of making lots more of these gems for all of you that have the fantastic Cobalto brakes but have lost the center stone. 
I also plan on making other colors of center stones (purple, pink, white, orange, yellow, Bianchi celeste, black, etc.) and if you have a particular color that you would like then please ask me.
 
Thanks