When dropped on ground, Moonlenses will appear on the Map for everyone in that world, serving as a marker, even in unexplored areas.
In case of the Purple and Blue Moonlens, they are also used for crafting the Ocuvigil and Moon Dial. In the beta branch, the Orange Moonlens is used to craft The Lazy Deserter. Each structure using a Moonlens to craft will keep its global map marker.
Prior to the A Little Fixer Upper update, Moonlenses had a different texture, being purely white with no shading.