Lines like this as well as smooth indents or bubbles do not indicate pontil marks found on handmade marbles.
Marble on microscope.
These are photomicrographs very thin slices of rock seen in plane polarised light or between crossed polarisers when the colours seen are produced by interference of light.
Some common rock types as seen under the microscope.
Marble width 2 8 cm uk virtual microscope the united kingdom virtual microscope ukvm collection consists of igneous sedimentary and metamorphic rocks from around the uk.
If the marble is completely smooth it was machine made and has no pontil marks.
Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism.
Check for smooth lines or a single straight line that runs deep into the marble.