Are you sure, mon ? Peltier black glass is especially susceptible to turning iridescent when buried, and that's what it looks like from here.
(Peltier clear glass often turns iridescent, also.
In the 1930's some of Peltier's glass formulas had a good percentage of recycled bottle cullet in the batch mix, and as a bottle digger, I wonder if that had anything to do with the iridescence on some colors.)