For those born without a sense of humour, Halopedia has an article on Glassing.
Marbleing, or glassing called by retarded humans, is the decoration of planets. The races of the covenant

are attracted to shiny shit so they Marble everything they see. They do this with glass shooters. Glass shooters shoot shiny shit at things. They range in size from personal shiny shit shooters to holy shit ship size shiny shit shooters. Since they are attracted to shiny shit they marble EVERYTHING. When the covenant came in contact with the humans they thought they were doing us a favor by decorating Harvest and its people with glass. However, they were surprized when the human shit-heads, lead by MR.T, attacked them. The Covenant realized that the dumb-ass gorillas must have done a piss-poor job of Marbleing Harvest so that the humans were not satisfied. A battle group was sent to look at the job the Gorillas had done. When they got there, it looked more like the gorillas had taken a GIGANTIC shit on Harvest instead of making it prettyful. The Covenant hoped to find other human worlds and people to make shiny and pretty so they could fix the King Kongs fuck-up. And so, the human covenant war started.

Glassing is also known as setting the planet on fire. The Covenant would invade a planet and start forest fires, pissing off all the bears. 

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