Should I buy stellar crystals with Diamond?

Hello folks,

im new to this game and playing it for 10 days. I already pulled 1 harak (also bought 1 harak with guild coins) and 1 pesto. I read that pesto is better for the begging than harak, I wanna focus on pesto now but i dont know if its worth to buy crystals with diamonds. I am f2p btw.