It currently is unknown when Earth’s dynamo magnetic field originated. Detrital zircon crystals found in the Jack Hills of Western Australia ranging up to 4.38 billion years in age might retain the oldest known records of geomagnetism. We have found that although the zircons' host rocks have not been heated above the Curie point of magnetite (Trail et al. 2016), these rocks have been pervasively remagnetized (Weiss et al. 2015, 2016) and the zircons themselves are contaminated with secondary magnetic minerals (Glenn et al. 2017, Weiss et al. 2018, Tang et al. 2019). Our studies of 0.767 million year old zircons from the Bishop Tuff in California nevertheless show that unaltered zircons can in principle accurately record the geomagnetic field (Fu et al. 2017).