What are Feltrite Crystals traded for in Rage?

Solution 1:

You can trade 20 of them to Jacob, he's in the bar in Wellspring, for a defib upgrade (which means you can die one more time before a game over)

There is virtually unlimited Feltrite in the game so selling a few doesn't matter. driving around the wasteland will occasionally activate missions automatically. One of the missions requires you to collect 7 falling Feltrites, afterwhich you keep all 7. I just recently came across another one where it was "collect as many as you can in the time given" - I got a whopping 15.

Note that the Feltrite shower events (Feltrite tempests/storms) are not unlimited, but scripted. There are 3. The first two will allow you to get up to 7 each. The third one will allow you to gain up to 20.

As far as any other use outside that one upgrade, I don't know of any.

Solution 2:

You can get an awesome upgrade later on for 20 crystals. Don't make the same mistake I did, save at least 20.