The flash is beautiful but why does it happen?
This is where my nerdy, engineering background is going to show through. I did some reading on mineralogy. I found out that the flash is called "Schiller effect". To understand Schiller effect, you need to be able to visualize how labradorite forms.
Labradorite is in a family of minerals called feldspars, which are classified by their chemical composition. Feldspars have a crystal lattice made up of silicon, aluminum and oxygen. In addition, depending on the type of feldspar, the crystal may contain potassium, calcium, sodium, or a mixture of calcium and sodium. Over 50% of the rock on Earth is feldspar.
The reason we aren't surrounded by gemstones is that most of these rocks cooled quickly. The molecules in the rocks are most stable chemically when they are arranged in large crystals, but the rapid cooling caused them to frozen in place before they could arrange themselves. Thus most rocks are made of a mishmash of microscopically small crystals. When feldspars cool slowly - for example the molten magma is trapped underground - there is more time for the molecules to arrange themselves into large crystals before they are locked into place.
Now here is the really interesting part: when a mixture transforms from a liquid to a solid, the result can be... complicated. The first crystals to solidify (at a higher temperature) have a different composition than the last crystals to solidify (at a lower temperature). In industry, this principal is used to separate out impurities and create very pure substances.
In any case, as feldspars cool and form crystals, the composition of the crystals being formed changes. In the real world, the temperature of the forming crystals can fluctuate up and down. This helps create layers inside the crystal with slightly different compositions.
Labradorite forms when the crystals contain a mixture of sodium and calcium. As labradorite forms into a crystal, the composition of calcium and sodium varies. The changes in composition forms layers inside the labradorite crystals. The layers inside the crystal affect light as it travels through the crystal. If you look at the layer from the right angle, you see the Schiller effect – the flash of color.