RuneScape Classic Wiki
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The main RuneScape Wiki has an article on: Lens mould
The Old School RuneScape Wiki also has an article on Lens mould.

A Lens mould is a quest item needed to complete the Observatory quest. It can be found in the gated room guarded by the Goblin guard in the Observatory dungeon.The Keep key must be obtained from the southwest most chest in the dungeon, and a Goblin guard either killed or ran past to get the mould that is hidden in the west most Sack in the gated room.

Players who search the sack will get the following message:

"Underneath you find a peculiar mould"
[If player has mould]
"You already have this lens mould
Another one will be of no use"

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