Caffeine is released during roasting. Some of this caffeine crystalizes on the outside as the smoke escapes.
This is a HotTop roaster. The yellow devices on top are temperature probes. As the roast progresses, smoke escapes from the probe holder. Over time, amazingly intricate white crystal structures appear. These caffeine crystals are know as angel hair in coffee jargon.
These are microscope shots of the crystals after removed from the probe.
Same cluster under higher power.