Could it be referring to a gem of seeing?
The gem of seeing does grant true sight.
According to the module's description and item list
"In my adventure The Priest, the Witch, and the Lost Temple, two new
magic items are featured: the Gem of Interdimensionality and the Orb
Gem of Seeing has been around for decades in various editions, and people have been mistakenly referring to it as Gem of True Sight for a long time, to the point where Defense of the Ancients made a similar item to gem of seeing and called it "Gem of True Sight". The more I research, the more it seems to be a very pervasive mistake, the DND equivalent of Nukular vs Nuclear.