She thinks it may have been an imported British or Canadian show. (NOT The Tomorrow People, because she used to watch that too and knows what it is.)
I'm hoping that someone may be able to help us figure out what show this was.
Two main characters, young brother and sister (twins?) who discover that they have special powers. Each character has (or later gets) a colored stone that helps him or her focus and use their power. Apparently, there are aliens involved somehow--a network of tunnels below the town is where these aliens hang out, and sometimes the characters end up running around down in the tunnels fighting the aliens.
Any thoughts? Even a possible would be good--we can look it up and see whether it matches her memories.
ETA: Thank you, jd3000! Roomie says that "Under the Mountain" is indeed what she was looking for, and she's very happy to find that it was just released on DVD. :-)