I have some disks of a doubtful quality and the first thing that struck me was the pattern of the burning surface.
With Atachne's help we described it as a moire pattern.
Basically it looks like a series of coocentric circles of 2-5mm thickness that have different shade of violet (2-3mm thick - purple violet, 5mm thick - more blue violet).
Is it something normal? I personally haven't seen anything like it before, and only came across fully uniform one color clean surface (unles there were stains etc.).
Arachne mentioned that some quality media also can have something like this...