Limestone and marble tiles can have lines in them that look like cracks. At first glance, this may concern you, but they are in fact veins of calcite which have the technical name of stylolites. Their presence is natural and does not reflect a defect or natural weakness in the tile – so good news, you don't have faulty tiles! The lines are an acceptable characteristic of natural stone tiles and they won't crack when walked upon.
So normal are they, that many porcelain tiles which are manufactured to mimic natural stone, are deliberately created with lines to authentically represent the calcite veins found in limestone and marble tiles.
The technical bit ...
The lines are ‘seams’ of calcite, with minor amounts of clay and minerals such as pyrite, which are formed when the limestone rock is subjected to great pressures and CaCO3 (calcium carbonate) minerals are dissolved and reprecipitated. The crystals of calcite interlock and form an important role in the compaction and hardness of the rock. They may extend laterally for up to tens of metres and may be sub-parallel to the bedding of the rock although in some circumstances they can develop perpendicular to bedding. Stylolites are visible only where less soluble minerals such as clay and pyrite are present in the limestone and become concentrated at the interface. Further, the varying colours seen in natural stone tiles result from natural impurities that mix with the calcium carbonate (CaCO3) mineral that all limestone is composed of. The lighter the shade, the greater the percentage of pure CaCO3 in the limestone. Metal oxides and coloured clays are common natural components of the limestone which impart colour to the rock.
Whether you've purchased natural stone kitchen floor tiles or bathroom wall tiles, all will have started out as large blocks of rock roughly 3m x 2m. Usually at source, these blocks are carefully inspected for cracks and flaws and tested for hardness, colour and durability. Blocks are transported to a manufacturing plant where they're are cut into slabs, and the slabs are cut into tiles. It's at this point that StoneStore pay independent inspectors to visit the plants and ensure quality is as expected, on passing inspection tiles are securely packed crates and shipped to our warehouse. Buying direct from the quarry by the container load ensures StoneStore control the process from quarry, to shipping, into our warehouse, and to your site.
Safe in the knowledge that tiles which appear to have cracks are not faulty or second grade, customers who view such tiles as less aesthetically pleasing ask their tiler to fix them out of sight such as under kitchen islands, or behind bathroom vanity units. Also, alert you tiler to the likelihood that a few of your tiles may have such lines and ask him to put them to one side and use them for cuts.
Before the tiler starts fixing your natural stone tiles you should expect him to check he has the correct tile, enough tiles to complete the job, and to do a 'dry run' at which point you can discuss how you'd like any tiles with the 'stylolite' integrated into the project.
One of the beauties of marble and limestone tiles is the unique variation from tile to tile, some will have darker tones, some lighter. To ensure there is an even spread of colour variance across the installation, we advise the tiler uses a process of 'shuffle and select' . That is, empty the crates into as many stacks as is practicable and select tiles from the stacks at random.
If you're not going to be on site it's prudent to leave the tiler the sample we sent so he has a guide to what tile he expects to work with and ask him to send you a photo of the 'dry run' for your approval.