It will look kinda crappy...
The glue, in contact with the back of the diamond will change the
optics of the stone, reducing it to the look of a gunky dirty
diamond such as one sees in jewelry that's been worn for years with
never a cleaning, not a bright reflective one. If you could keep
the glue to only be in contact with the girdle, it would look
better, but this will be quite hard to do. I'd suggest setting the
diamond in a tube, in the normal bezel/tube setting manner, and
gluing the whole tube into the hole in your glass, keeping the
diamond free of any contact with glue. THAT will work just fine. If
the hole in the glass goes all the way through, it even allows you to
easily clean the diamond later. if it doesn't, then be sure the
bezel /tube edge is burnished down really tight to the diamond,
sealing it well, and be sure the glue is also fully sealing the tube
in, without gaps. That way, perhaps dirt and grease, (etc) won't get
behind the stone.
Also, be aware that the longer setting-time epoxies are stronger and
longer lasting, and tend to be more moisture resistant over time.
Devcon 2-ton is OK, but I prefer the "epoxy 220" or "epoxy 330"
types from Hughes. They're made specifically for gems, metals, and