This card shows a young woman observing some stalactites in the Cave of the Mounds.
Regrettably, the black and white photo, combined with the limited lighting in the cave, result in a photo that is not particularly evocative of the name. As a result, it is not clear why this is the Gem Room, other than the fanciful imaginations of the owners and marketers. The description states simply:
Gem Room – Cave of the Mounds, Blue Mounds Wisconsin
The woman’s attire indicates this photo is from the 1940s.