When I was farming them, as I exited Cetus, I took a hard left, hit Twin Horns lake to check for gems and Wisps, then head to Er-Phyrgal (spelling?)

there is no spot except Er-Phryah's Vigil, which has the densest location of gem spots in the game. just run that area with an advanced nosam cutter, and both resource boosters then extract then repeat.

without the booster and advanced cutter, it will take you a while to get any amount of rare gems and it would be faster just to farm plat and buy the gems from people

I remember reading this from some comment on the wiki and it been working for me quite well. It pure superstition, but if your looking for sentirum then I found going along the right side of cetus then up the waterfalls yields quite alot of sentirum. If you bring a frame/companion with loot radar or animal instinct you can also farm cetus wisps as well.