This appears to be a limestone fireplace, yes.

Agreed, this looks like a cream or buff limestone. I really like the general design of this piece.
However, I have to point out a significant error of judgement on the part of the manufacturer.
The uppermost pieces of the bracket jambs show heavy striations running vertically. When designing and fabricating an architectural element using stone these striations should always run horizontally.
The reason being is that if there is a flaw in the stone and a failure were to occur, it will happen along those striations lines. If they are horizontal then the failure is 100% without consequence and would probably never even be noticed. But, if the striations are running vertically, the failure would possibly result in part of the piece sheering off. Not a desirable scenario, to say the least.

Please note: that this is all extremely unlikely, but good design and production should always take the worst possibilities into consideration. It's a good example of why it is important to find a knowledgable and expertly trained craftsman to design and fabricate your architectural details and elements. Find out more about me at: http://www.bartlettstone.com/home

This looks like a 68-72" Wide x 50/52" High. Opening looks like a 42" Wide x 24-26" High