I find it very unlikely that a soul stone is actually required for Astrologian, otherwise Leveva would require a mountain of them in order to maintain a teaching center for Sharlayan Astromancy in Ishgard. I'd think its safe to say that it may be required for the most powerful Astrologian spells, but the more basic ones would be doubtful.
“...most people need to believe in something. There are a few who don't, but they're a bit unusual.â€Â
-The Redemption of Althalus
“Sparhawk grinned. "If Martel finds out that he's drinking again, he'll reach down his throat and pull his heart out."Â
"Can you actually do that to a man?"Â
"You can if your arm's long enough, and if you know what you're looking for.[...]â€Â
-The Diamond Throne
-The Redemption of Althalus
“Sparhawk grinned. "If Martel finds out that he's drinking again, he'll reach down his throat and pull his heart out."Â
"Can you actually do that to a man?"Â
"You can if your arm's long enough, and if you know what you're looking for.[...]â€Â
-The Diamond Throne