First of all congratulations on your recent purchase. If you cut the line, I would recommend you replace the whole section from the Tee to the appliance, or first valve if possible so as to avoid having any more connections than nescessary.
It should be copper, though I'm not sure of the line size ( I'd guess 0.375"). You'll want to use flare connections.
"You wouldn't worry so much about what others think of you if you realized how seldom they do."