When I have found these long sliver shaped crescents in the past, I have made them into complete necklaces, attaching a chain each side. Since so many people already have spic chains in their collection, often watch chains which can be on the shorter side, I thought an extender would make sense. 
 
This Late Victorian - Edwardian piece was a brooch, mounted in silver and gold, and set with bright, lively old European cut diamonds, perfectly graduating to fit the crescent shape. We have added recycled  gold loops to either end, large enough to easily accommodate your dog clips or bolt rings. It will also work beautifully with a chain running through both loops. 
 
Tested as silver and 18ct gold 
Weight approx 4.22g 
Width (measured straight across) approx 70mm, but will effectively add a little more length to your chain, accounting for the curve. 
Diamonds tested as natural, earth mined diamonds
Approx G-I colour, and VS-SI clarity
Total calculated weight approx 1.20ct 
Excellent antique condition