Glassing

Glassing is a technique used by the Covenant in which they map the latitude and longitude of a planet's surface, divide it into grids, and bombard each grid with heavy plasma fire, thereby obliterating all life on the planet and turning its surface to glass. After the glassing is done, the planet is a red smoldering sphere. After a few days, the atmosphere of the planet supposedly corrodes and dissipates, leaving it truly barren.

This technique was used to obliterate the UNSC outer colonies and Reach. After they glass the planet, they remove a shard of glass and take it to the Step of Silence in High Charity where it hangs with hundreds of other shards from other planets.