This is real triboluminescence, not folklore: wintergreen (sucrose) crystals are non-centrosymmetric — they lack inversion symmetry. When a crack propagates through such a crystal, the newly created fracture faces carry opposite bound charge (a piezoelectric-like separation, exactly as squeezing quartz generates a voltage). The charge builds a strong local electric field across the microscopic gap between the two faces as they separate.
Dry air breaks down (avalanche-ionizes) at roughly 3 MV/m. Once the field across the fracture gap crosses that threshold, electrons avalanche through the gas, producing a real electrostatic-discharge flash — the same physics as a miniature lightning bolt. In wintergreen candy the UV component of that flash is absorbed and re-emitted as visible blue-green light by methyl salicylate fluorescence, which is why wintergreen Life Savers glow more vividly than plain sugar.
- Asymmetry — sets how much bound charge each fracture separates; more asymmetric crystal lattices (and higher-strain-rate breaks) shift the flash toward energetic blue-violet.
- Humidity — a moisture film on the crystal bleeds separated charge away before it can discharge, which is the real reason triboluminescence demos are done with dry candy. It does not change air's breakdown field, only how much charge survives to reach it.
- Darkness — the flash itself doesn't change; a dark room is simply required to see a discharge this faint, exactly as with real wintergreen-candy demonstrations.