Hey! Thanks for playing. You can now pull the limited banner with Gems if you're out of Crests, but it's a steep 500 Gems per pull (5x the standard cost). I went with 5x instead of 2–3x because I still want Solar Crests to be the real currency for limited stuff, but I didn't want you to be totally locked out if you're dry on Crests.
Basically: if you have Crests, it uses those. If you hit 0, the button flips to Gems automatically.
Thanks for the suggestion; those fixes will be published with the next major update! <3☀️