Gemstones for the lunar nodes are selected more strictly than those for the planets, and they're almost never prescribed first.
The position of Rahu itself is what's examined: which house and which nakshatra it occupies, which planets it's conjunct with, and which areas of life it touches. Rahu doesn't rule any sign, so it's read through its surroundings — through the planet whose sign it sits in, and through its connections to the other planets.
There's no averaged answer here, only a particular chart.