Chau, cast cement stones (such as fibrament) tend to have moisture in them from the casting process. If the stones have water in them or are stored/shipped in an environment where they get wet or are exposed to humidity, they'll crack if you crank up the heat. Sure, the company will replace them, but if it's between a couple of hours in a home oven vs. the potential of having to wait for the company to send me new stones, I might toss it in the oven. Am I being overly cautious? Probably- but porous stones are notorious for failing due to moisture expansion- and these, by their density, are incredibly porous stones.
Btw, when I talk about an oven, I'm talking about an indoor oven. I believe 3/4" fibrament is a 5 hour process, but, at 1/2" I think 2 would suffice- one hour at 150 and then another hour at 300. Turning the oven on right now, with the temps we're seeing, is probably something most people would want to avoid, but I'm just telling you what I would do.