The Diamond Play Button, is awarded for channels that surpass 10,000,000 subscribers in YouTube. It is made of silver-plated metal inset with a large piece of crystal in the shape of a play button triangle.
Out of the four tiers of rewards, only the Ruby Play Button, which has only been awarded once for channels that surpass 50,000,000 subscribers, is on top of the Diamond Play Button!
For your further information, below is the video clip for unboxing the Ruby Play Button :
This is just one of the 4 awards by YouTube, viz. Silver Play Button, Gold Play Button, Diamond Play Button, and Ruby Play Button, for recognising channels that have surpassed a certain number of subscribers, 100,000 for Silver, 1,000,000 for Gold, 10,000,000 for Diamond, and 50,000,000 for Ruby.