Faden Quartz crystal tab from Pakistan.
Faden means thread in German. This formation of tabular quartz has a white line or string running through it.
These crystals formed in the holes of rocks in situations where they were repeatedly broken and reformed as they grew. As broken edges came back together in the reformation, liquids and gases in the vicinity got incorporated into the newly formed crystal. They enclosed liquid and gas created the white thread or line in the middle of the crystal.