= Solution
An unstressed <glass> is optically isotropic, so it does not split incident light into differently phased in-plane components; it remains dark between crossed polars. A natural explanation of the colored regions after heating is <devitrification>: anisotropic crystallites have nucleated in the <glass>. Edges and surfaces provide <heterogeneous nucleation> sites, making their greater concentration there plausible.
Each approximately uniform crystallite can give a nearly uniform <interference colour>. Regions with similar thickness and effective <birefringence> have similar retardation and therefore similar spectral colour, while differing azimuths multiply the intensity by $\sin^2(2\alpha)$ and change brightness. Identical hue does not prove identical three-dimensional crystallographic orientation.
Stress-induced <birefringence> is a possible alternative in a heat-treated <glass>. The observations alone do not uniquely prove crystallinity; rotating-stage extinction, morphology or diffraction could distinguish the explanations. The crystallite interpretation is the straightforward phase-transformation explanation, without inventing a composition or heat-treatment schedule.
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