Identification of quartz Brazil Law Twins is difficult and is done visually by identifying crystal growth and etching patterns on crystal faces. Also, the beautiful iris effect (a rainbow display of colour) in agates is due to Brazil Law Twinning of the chalcedony in the fortification bands that disperse white light into a rainbow of colour. Experiments with crossed-Nichol prisms (use of polarizing effects) show a distinctive zigzag pattern in thin-sections of Brazil Law Twin quartz. This pattern is called “Runtzel Banderung”, after the German physicist who discovered it.