In the past, you could forge a Darkness Gem with a Hollow Stone (along with 2 Stamps or CCPs) to create a Night Stone, but here, when you do the forge, you can ask for a Dark Stone alternatively.
The Night Stone evolves Eevee to Umbreon, and the Dark Stone evolves Misdreavus into Mismagius, or Murkrow into Honchkrow (though, in the games, this stone is known as the Dusk Stone instead).
Of course, if you didn't need the Darkness Gem or any particular gem, you can always sell it for 2 Stamps or CCPs; believe me, you'd probably be doing it a lot if you got a lot of gems here.