Sentinel Crystals are Utility Crystals of Luminoth design. They monitor key areas of Aether and broadcast information that U-Mos still uses to keep an eye on Ing activity. Their already durable design was improved upon for use in Dark Aether after many of the original Sentinel Crystals were destroyed and these adapted versions, called Dark Sentinel Crystals, were installed in enemy territory.
- "The original Sentinel Crystals were durable, but they had their limits. The toxic nature of Dark Aether, coupled with constant attack from the Ing, destroyed many of them. The Luminoth responded with the Dark Sentinel Crystal. Sheathed in dark energy, it is more resistant to the atmosphere of the Ing homeworld - and the attacks of its denizens." —Logbook Data (Metroid Prime 2: Echoes)
- "During the war, the Luminoth used the Sentinel Crystals to monitor key areas and installations. They are incredibly durable, resisting all but the most powerful of Ing attacks. U-Mos still uses the remaining Sentinel Crystals to keep a watch for Ing activity." — Logbook "Sentinel Crystal" (Metroid Prime 2: Echoes)