Grouting for fireplace chamber?

[True North]

Hi, I've tiled the inside of a fireplace chamber, off-white herringbone marble. A labour of love but that's another story.
It'll eventually have gas stove fitted, no open fire.
Have used heat-resistant adhesive, what should I be looking for when buying grout. Only white needed.

Thanks much.

 

[parkerfresh]

Mapei mapeflex firestop is an option, suitable up to 1200c. Comes in a silicon style tube and can be piped in like mortar.

Decent regular grouts like a Bal micromax is only suitable up to around 150c