This descriptions does not sound like it's about something that is "virtually unbreakable" : "The sample looked like an ordinary piece of glass, 4 inches square and transparent on both sides. It’d been packed like the precious specimen its inventor, Adam Khan, believed it to be—placed on wax paper, nestled in a tray lined with silicon gel, enclosed in a plastic case, surrounded by air bags, sealed in a cardboard box—…
We actually broke low quality diamonds with a hammer. They are also very easy to burn.
What is very hard is scratching diamond. If you combine the properties of different materials like diamond on the surface, and something that absorbs impacts inside it could be a wonderful material.