In order to strengthen these stones to be used as countertops, they undergo a process called resinization, where epoxy resin is used to fill the weak spots of these stones. These slabs, with the visually filled lines, do not break or crack once they are installed, with proper care. If any problem does happen, it will happen during fabrication or transportation.
Mascarello, Copenhagen, Louise Blue, Barricato, Betularie, and many more popular stones belong to this category. A lot of customers do not mind the fill lines and visual imperfections because of the beauty of these natural stones.
If you are a perfectionist, and do not want any imperfections, then you should choose another type of stone.