Frame 3 way miter cuts are just glued, need accurate cuts to fit well and tricky glue up. I used a drop of CYA with wood glue and held against a square to position parts and tape to hold in place.  The CYA holds well untill wood glue sets.  Probably not strong for a larger piece but this was my first attempt.  I made a couple test parts first and could not separate the glued up joint by hand.  
And question about the diamond inlay,  I used a double bevel cutting method on scroll saw if you are familiar with that, to get tight fit.  The maple and bloodwood diamond are about 1/8 inch thick.

TimV, "The understanding eye sees the maker's fingerprints, they are evident in every detail, leave Fingerprints." James Krenov