Because the Blu-ray standard places data so close to the surface of the disc, early discs were susceptible to dust and scratches and had to be enclosed in plastic caddies for protection.
The solution to this problem arrived in January 2004 with the introduction of a clear polymer coating that gives Blu-ray discs unprecedented scratch resistance. The coating, developed by TDK Corporation under the name "Durabis", allows BDs to be cleaned safely with only a tissue — a procedure that can damage current CDs and DVDs. Presumably HD DVDs are similarly frail, as they are manufactured by the same process as the older optical media. Bare BDs with the coating are reportedly able to withstand attack by a screwdriver. http://en.wikipedia.org/wiki/Blu-ray_Disc#_note-5"> TDK have used the same coating on their current range of "Scratchproof" DVD media. Recently, TDK has made the DURABIS2 coating, which can withstand being grinded with both Sandpaper and steel wool. It will be used on the Blu-ray exclusively.