I think you will have your work cut out to get anything like an invisible repair. The traditional method would be to set in an indent or in other words splice in a new piece of stone.
As to filling if you want to give it a go then something like this may be worth a shot http://stores.ebay.co.uk/Fascinatin...=9182505&_sid=173514689&_trksid=p4634.c0.m322
there's a few different colours here.
An alternative may be to cover it up someway with something like a oppositely coloured tile.
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