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.
Marble on microscope.
Lines like this as well as smooth indents or bubbles do not indicate pontil marks found on handmade marbles.
Check for smooth lines or a single straight line that runs deep into the marble.
If the marble is completely smooth it was machine made and has no pontil marks.
Some common rock types as seen under the microscope.
It is composed primarily of the mineral calcite caco3 and usually contains other minerals such as clay minerals micas quartz pyrite iron oxides and graphite.
They are shear lines caused by machines and are not true pontil marks.