Yes, crux is a Scrabble word!
...and is worth 13 points in Scrabble, and 15 points in Words with Friends
crux
Noun
1. A small conspicuous constellation in the southern hemisphere in the Milky Way near Centaurus
2. The most important point
What Scrabble words can I make with the letters in crux?
3 Letter Words
2 Letter Words