Dw 1620

hi, i recently bought a samsung dvd burner 16x drive and dw 1620 but when i’m burning 8x, both take about 8:05 mins which it’s really good. i’m using verbatim dvd-r 8x and when i finished burning i flip it over and look on the disc under my lamp i see coasters, but only under a bright light, it reads perfectly fine though on my dvd-rom, dvd players or whatever, no skips or nothing. i was just wondering, would the disc be bad after a few months or playing, or what exactly do coasters do? i have a p4 3.2ghz with 1gb ddr 3200