Found another one in TF. The Cavern of Glass is not set up for mining either, but is in Prime.

> [prospect]: look
[Aldauth's Lair, Cavern of Glass]
Shadows flicker on every wall possibly coming from the torches reflecting off of the black crystal walls. Tunnels branch off in many directions and show no signs of ending. An unfamiliar sound, possibly hissing, is enough to make hair stand on end.
Obvious exits: east, southwest, west.
> [prospect]: prospect
You scan the area and determine that it is not suitable for mining.
Roundtime: 5 sec.