Forerunner planet creaton

"Our ship stopped at several installations where the science of planetary formation was being taken to advanced stages. Rocky worlds were at a premium, one of the Miners explained to me in the ship’s small, sparsely appointed lounge. Forerunners now had the ability to collapse an asteroid field into a molten mass of the twenty-megameter range, then cool and cure the protoplanet in less than ten thousand years."
(Halo:Cryptum; Ch.27)

Onyx shattered and the surface exploded into space. Obscured by layers of dust and fire, a blazing pattern emerged beneath: crosses and lines and dots.
"Magnification factor one thousand," Lash ordered.
Yang was frozen.
Waters bent over and tapped in the command.
The view on-screen blinked and stepped closerpast boiling air, clouds, tumbling mountains zooming to ground level, revealing a lattice of three-meter-long rods and half-meter blazing red spheres that hovered between them, forming a crystalline structure.
"Back it off," Lash said
The view pulled back and showed that this drone-constructed scaffolding
stretched over kilometers they had been under every landmass, every ocean under the entire surface orderly linked rows like
the carbon bonds of an infinite polymer chain, or an immense colony of living
interlinked army ants.
The drones were the planet Onyx.
"There are trillions of them," Lieutenant Durruno whispered.
Clusters of drones heated; culminated beams shot forth again, targeting more
distant Covenant vessels and vaporizing them.
(Halo:Ghosts of Onyx; Ch.40)