Cracked Marble Fireplace Surround

Hi everyone.
I'm a newbie to the group.
I had my mason replace a gas fireplace with a real masonry wood burning one.
Opening is 40 by 29 with a 12" by 12" flew.
My problem is this.
Had the marble fabricators install (4) 12 inch wide 3/4" deep Jerusalem Gold around the fireplace opening.

The top piece developed a small crack from top to bottom in the middle. They replaced it with another piece this time with 2 stainless steel
strips embedded lengthwise in the back of the top piece.
It ALSO cracked.
My question? Is the fireplace burning too hot - I measured it at 185 to 190 degrees in the middle top area (with infrared temp. device) OR is the marble
they are using from an inferior source - should this type of marble be able to withstand 190 degree heat over several hours time?
Any and all suggestions will be appreciated.
Thanx,
bboomer@optonline.net

Is it real marble or an agglomate?

How are they supporting the marble? What is the substrate? Glue, mortar, etc?

Another question why should it be 190 degrees that sounds excessively hot Is air space required been covered up?

They applied the marble against the wall/brickwork with (I believe) thinset.

And I am wondering if 190 degrees is normal for the top wall area over the fireplace opening?

Again thanx for any advice/comments, etc.

Brett

The air space seems fine.

I have a 12/12 flue with a proper air chamber.

I will tell you I had to raise the grate by 2.5" (with cut brick at each foot) in order to get good air flow, so as to keep the fire burning properly.

BB