Did I ruin my stoneware dutch oven by heating it on a gas stove?
It sounds like you had a lucky escape. It's not temperature itself that would damage stoneware as it's fired at over 1000C in manufacture (Wikipedia). Differential thermal expansion is what breaks things, e.g. heating the base much faster than the sides, or heating a thin layer quickly before the rest has time to warm through. By preheating it slowly you probably avoided that. 

I would however be a little wary for one or two uses - handle it carefully and use it when you wouldn't be devastated if it broke and ruined the meal. Also inspect for cracks. 

The temporary darkening is a bit odd but unlikely to mean anything.