yep. sa tempered glass, ganyan. something to do with the composition of the glass.

hope this link helps:

http://en.wikipedia.org/wiki/Tempered_glass

The pattern of cooling during the process can be revealed by observing the glass with polarized light, which shows the strain pattern in the glass.
i don't know if this was relevant. but it is common for the tempered glass to have this kind of effect (eto yung 'rainbow effect' na sinasabi).