From Halopedia, the Halo wiki
Nanolaminate is a type of material used as battle plating for Covenant warships and Sangheili combat harnesses. Possessing a characteristic opalescent-like shine, nanolaminate can range in color from purplish to white-blue or silver, while Kig-Yar vessels are generally bluish-black in color. The plating is very strong and heat resistant, with the support of energy shielding allowing Covenant ships to take more damage. The amount of nanolaminate plating applied to each ship depends on the class, though Ruma-pattern light carriers have well over thirty meters of armor plating. As of 2525, nanolaminate plating has not seen use in any human construction.
Nanolaminate hull plating allows ships to be resilient to most UNSC weaponry. UNSC point-defense guns are not powerful enough to do significant damage to the plating, Archer missiles prove to be very effective against the unshielded hull of a Covenant vessel, causing the ship to become very vulnerable following the resulting explosions of the missiles. Magnetic Accelerator Cannons are often used to disable and destroy Covenant vessels after the energy shielding fails, being powerful enough to pierce the nanolaminate hull plating of most ships with a single round. A single pack of C-12 shaped charges can easily puncture a large hole through the hull of a Covenant cruiser. Some Covenant cruisers are capable of taking a direct hit from a mass driver at near point-blank range with only minor structural damage done to the nanolaminate plating, but a second hit in the same spot will puncture the vessel.
However, Covenant weaponry is more effective against nanolaminate plating. The carrier Ascendant Justice, one of the most powerful vessels in the Covenant fleet, was heavily damaged by a single plasma torpedo, with several layers of the armor plating boiling away.
The Sangheili continued to improve their ships' nanolaminate plating and other onboard technologies in the wake of the Human-Covenant War. Following a joint anti-piracy operation with the UNSC Navy in a Joint Occupation Zone in 2557, ONI analysis noted improvements to the hull plating of the assault carrier Shadow of Intent.
The nanolaminate plating is very heat resistant, allowing Covenant vessels to enter extreme atmospheres or absorb the long-range damage from human nuclear weapons. Upon entering the gas giant Threshold's atmosphere shield-less, the hull of Ascendant Justice heated to three hundred degrees Celsius, with no actual structural damage done to the ship. Only when the ship reached temperatures of over seventeen hundred degrees Celsius did minor hull tearing and conduit melting occur. However, at such high temperatures, the superstructure of the vessel becomes considerably softened.