Did I ruin my stoneware dutch oven by heating it on a gas stove? I was following this baked beans recipe. Relevant excerpt:

Place a cast iron Dutch oven over medium heat and stir in the bacon, onion, and jalapenos until enough fat has rendered from the bacon to soften the onions, about 5 minutes. Stir in the tomato paste, dark brown sugar, and molasses.
Drain the beans and reserve the soaking liquid. Add the drained beans to the Dutch oven. Place the soaking liquid in a measuring cup and add enough vegetable broth to equal 4 cups of liquid. Add the liquid to the Dutch oven and bring to a boil over high heat.

I don't have a cast iron dutch oven and wasn't sure everything would fit in my cast iron pan, so I started the process in the pan and transferred everything to my stoneware dutch oven (smooth and shiny - possibly enameled?) once it was time to add the beans and broth.
I had preheated the liquid in the dutch oven on low on my gas stove, and once I added everything else, I cranked it up to high. After a few minutes the bottom of the dutch oven darkened quite a bit, so I googled and found that you're not supposed to put stoneware on a direct heat source. I turned off the heat and transferred everything back to the pan (where it thankfully just fit) and resumed the recipe from there.
The dutch oven returned to its normal color once it cooled off, but I know that not all physical changes are clearly visible. It probably spent 5-10 minutes on low and 5-10 minutes on high.
Is the stoneware now significantly more likely to crack when heated? Or because it survived the endeavor, is it still okay to use it normally?
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.