Open fireplace - Marble Hearth cracked

I got an open gas fireplace converted to an open fire. It has a marble hearth, and has a 1mm split down the middle already to allow for expansion.

I was planning on only using logs and turf in it, as advised by the installer, so that it doesnt get so hot that it cracks the hearth, but ended up putting some coal in it anyway and I have now noticed a crack.

What shoudl I do to minimise the damage here?
Its not noticible really, its a hairline crack, but I dont want it to get any worse.

Any thoughts?

The exact same thing happened to both my marble hearths shortly after I used them but that was about 6 years ago however they have not got any worse. Both still have a crack down the middle and I use them all the time