Quartz crystals inside the host rock will rupture when the fissure opens. In a silica rich solution, this rupture will heal quickly, forming a crystal that is attached to the opposing rock walls and bridges the new opening. While the fissure continues to open steadily, the crystal will also continue to crack and heal. They are found in areas of low grade metamorphism in which cavities in the rock are growing. When these zones grow, the quartz crystals in them are broken (repeatedly), and the healing and regrowth incorporates fluid and gaseous inclusions ( the white threads). The edges of the crystals grow faster and incorporate the liquid inclusions in the center of the crystals. Read more here: https://www.geologyin.com/2018/05/what-is-faden-quartz-and-how-does-it.html