That's a nice looking pie, Mark. Baking time?
I did a little research on your stone. Basically, stone identification can get pretty imprecise.
To begin with, the term 'soapstone,' in itself, is not designated scientifically. Someone, years ago, rubbed a stone containing a high quantity of talc, and, based upon the soapy feeling it gave off, coined the term 'soapstone.' Over the years, 'soapstone' has taken on a very wide meaning. The kind we use/need, the variety with the necessary thermal characteristics, is pretty narrowly defined (dark gray/powdery), but other stones have wedged their way under the umbrella. One of these stones is serpentine:http://en.wikipedia.org/wiki/Serpentine_group
If you go to the bottom of the page, you'll see a photo of polished serpentine that's almost identical to yours. I initially referred to your stone as 'green marble.' Many people classify serpentine as marble. The owner of one of the larger soapstone distributors (Teixeira) talks about some of the confusion over the use of the word here:http://ths.gardenweb.com/forums/load/kitchbath/msg072015087866.html
(towards the bottom of the page).
That same forum has a discussion where the differences between serpentine and soapstone are gone over:http://ths.gardenweb.com/forums/load/kitchbath/msg0422251010723.html
Confused yet? I know I am. I think there's three factors that are clouding this issue.
1. Mother nature doesn't use recipes. She makes stones out of whatever is around. A particular vein of serpentine might actually have some talc in it, and, when you start adding talc to stone, you enter in the soapstone realm.
2. Soapstone is very rare and very expensive. Serpentine, not so much. Serpentine is also very pretty and, although slightly green, is closest in appearance to soapstone of all of the marbles.
3. Soapstone's rarity translates into very few stone suppliers being aware of what it looks like. From what I can tell, there's only one outfit in your area that could truly identify soapstone and that would be Creative Soapstone (Punta Gorda)- and they're no guarantee as they may include serpentine under the soapstone umbrella.
I'm going to give your stone suppliers the benefit of the doubt and lean towards ignorance rather than fraud.
Regardless of whether or not serpentine can be classified as soapstone, the stone that you purchased does NOT possess the necessary thermal properties for baking. You've already begun to see this by the amount of time it takes to preheat. If your oven goes to 550, soapstone should have absolutely no problem reaching that temperature in an hour. Marble/serpentine are in the 2-3 W/mK realm of thermal conductivity, while soapstone is 6.4. A less thermal conductive stone will take longer to preheat and will transfer less heat to the pizza resulting in a longer baking time.
One of the most important aspects of conductivity is that it allows heat to travel through the stone in a more even manner. Uneven heating usually translates into uneven thermal expansion. Uneven thermal expansion, in turn, results in structural failure. This is why you never ever (ever!) want to bake with marble (or granite, or slate, or any other rock that isn't soapstone). It's not a question of if it will crack, but when. In the high temps of baking pizza, it will be quick. And then your gorgeous $75 stone, that could have been used to temper chocolate (or other applications that utilize marble's properties/beauty), will be toast.
Take the stone out of the oven and never heat it again. Track down the real deal (See photo below). The left side is treated with mineral oil (you almost never find it like this). You want a gray, powdery, talc-y, soapy feeling stone with absolutely no shine.
P.S. Although I did say I was 99% certain before, and now, with the heating characteristics, I'm 99.9% certain that you're working with the wrong stone. Just to be 100% sure, could you take a few more photos of the stone? Front and back, in daylight, would be helpful.