This nasty device can take the form of any small item, but most often appears as a small figurine carved of jet or other black stone. As soon as the mine is taken into an extradimensional space, such as that created by a rope trick, portable hole, or bag of holding, it ruptures that space. Everything in the space, including the mine itself, is spewed into the Astral plane and is lost unless someone can retrieve it. If the extradimensional space was created by a magical item, that item is destroyed.
This doesn't happen in Sigil, where the only way to reach another Plane is through a Portal. Extradimensional spaces still work there, however.
Source: 2nd edition AD&D Tome of Magic, page 136
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