This brooch is made from Sterling silver, titanium and copper.  As Titanium can not be soldered, this piece is put together by riverting method.
 
Measurement is approx. 50mm x 30mm
 
The rainbow colour of the titanium is achieved by heating the metal.  It started as a grey colour metal that has to be cleaned properly to make sure there is no dirt or oil on it.  The colour will gradually appears by heating the metal with a torch.  It started with the dark blue  and the longer we heat it up, the lighter the colur become (that is how we get the rainbow effect by controling the heat). 
One thing to remember while heating up titanium is that we can not over heat it otherwise it will turn back to grey and we have to clean the metal properly again (it involves cleaning it with very fine sandpaper and wiping/washing it with methylated spirits to remove all grease/finger marks)