You take a few lasers pointed in from different angles and have them cross at a single point. Individually they don't have enough power to damage the material but they do when you cross multiple beams
Hmm I would have thought they were a special kind of laser:
ultrafast lasers, or femtosecond lasers.
They have extremely short pulses with huge amounts of peak power. Unlike 'normal' lasers that machine or mill material away by boiling and vaporising, these lasers machine or mill by 'cold ablation': the huge light field power frees the electrons, then it becomes opaque, so absorbs more light, so it frees more electrons, then it becomes more opaque etc etc. Eventually there is no more electrons holding the material together, and it just explodes.
It's just that it happens in a tiny scale; if you look at it with a microscope, it's an absolute mess.
They're called [sub-surface laser engravings](https://www.youtube.com/watch?v=sOrby692Uag
) or bubblegrams. When doing laser engraving, typically they focus the laser on the surface of the material. It creates a microscopic fracture. With glass, crystal, and certain plastics, you can focus the laser below the surface. This causes the microfracture to occur deep within the glass itself, not just at the surface. By adjusting the focus depth, as well as the X and Y coordinates of the laser, a 3D engraving can be made.