A glossy lamp is an experience lamp giving 110k experience in the Slayer skill, awarded to the player upon completion of The World Wakes quest by the Guthixian High Druid. This lamp requires a Slayer level of 70 to use.
There currently is a bug where you can receive the lamp even though you do not have the required Slayer level. You will not be able to bank the lamp neither can you use it. However if you destroy it you will later be able to reclaim it.