I found this in the Guardian Weekend magazine :
"Q.
...the hearth cracked in half right up the middle ... The two pieces sit together relatively neatly, although there is a gap where the edges of the crack have crumbled slightly."
A.
We asked Henry Masterton of period fireplace specialist Chesney's (020-7627 1410, chesneys.co.uk) for a DIY solution. He says the best way to repair [it] is as follows.
Place the two halves upside down on a flat surface. Make indentations in the marble, where steel rods can be placed to help strengthen it where it has broken. Use Mastik (a specialist glue designed for use with marble and available from any hardware store or builders' merchants) to secure the rods, then turn over the hearth and use the marble from the slots - crushed and ground with a hammer, then mixed with superglue - to fill the crack. When the filler is set, sand down the hearth and polish to the required finish.
That said, however, marble often breaks because it has calcified and lost its structural integrity, in which case it might be better to replace it with a fresh piece of slate, stone or marble."
"Q.
...the hearth cracked in half right up the middle ... The two pieces sit together relatively neatly, although there is a gap where the edges of the crack have crumbled slightly."
A.
We asked Henry Masterton of period fireplace specialist Chesney's (020-7627 1410, chesneys.co.uk) for a DIY solution. He says the best way to repair [it] is as follows.
Place the two halves upside down on a flat surface. Make indentations in the marble, where steel rods can be placed to help strengthen it where it has broken. Use Mastik (a specialist glue designed for use with marble and available from any hardware store or builders' merchants) to secure the rods, then turn over the hearth and use the marble from the slots - crushed and ground with a hammer, then mixed with superglue - to fill the crack. When the filler is set, sand down the hearth and polish to the required finish.
That said, however, marble often breaks because it has calcified and lost its structural integrity, in which case it might be better to replace it with a fresh piece of slate, stone or marble."