Marble is a natural material formed from heat and pressure. Each piece of marble is unique. One characteristic of marble is fissures or veining.
The veins or fissures give marble a natural artistic look.
Fissures are visual separations along intercrystalline boundaries.
In this image, you will see an example of green marble.
The white lines or veins are fissures in the marble. This is normal for most marble and some marble may have more fissures than others.