Geologists are studying about ten quarries north of Paris to find the same limestone as that mined for the cathedral underneath the French capital. That vast network of quarry tunnels beneath Paris was closed in the 18th century as collapsing galleries created dangerous sinkholes in streets and fields around the city.
Workers need new stone to rebuild collapsed parts of the vault and replace blocks in the gables over the three main entrances and other sections high up the side walls that were weakened by the fire.
Experts have been carefully testing the strength of surviving stones with everything from radar beams to simple mason’s hammers. Some stones that fell from the vault can be reused, but others have lost their inner solidity.https://www.thetablet.co.uk/news/13834/french-experts-search-for-suitable-stones-for-notre-dame