Many silicate minerals have one, or more, planes of weakness in the mineral structure called cleavage planes. As the name suggests, these planes cleave easily along a very smooth surface. The mineral that displays this the best, and is familiar to most, is mica, which splits into very nice little sheets. This surface makes a nice reflective surface when the sun hits it at the right angle.

Given the lack (?) of lower temperature felsic rocks that host mica, this mineral is unlikely to be our suspect. Mafic rocks, such as basalt and gabbro, do contain plagioclase feldspar, which has two cleavage planes at nearly right angles to each other. When looking for feldspar in a hand sample, it helps to turn the rock in the sun and look for little sparkles coming from the cleavage planes.

So that's my first guess: a phenocryst (a larger crystal) of sodium rich plagioclase feldspar, specifically labradorite, which also displays an irradescence due to light bouncing around internal crystal twinning planes. (That's a longer discussion.)

OR, it might be a piece of volcanic glass, like obsidian, which would also not be unexpected in this setting. However, glass breaks along a scallop shape (concoidal fracture) so the reflection tends to sweep across the surfae as opposed to being a single plane.