Russian measurements of the quality factor (Q) of sapphire, made 20 years ago, have only just been repeated in the West. Shortfalls in tacit knowledge have been partly responsible for this delay. The idea of `tacit knowledge', first put forward by the physical chemist Michael Polanyi, has been studied and analysed over the last two decades. A new classification of tacit knowledge (broadly construed) is offered here and applied to the case of sapphire. The importance of personal contact between scientists is brought out and the sources of trust described. It is suggested that the reproduction of scientific findings could be aided by a small addition to the information contained in experimental reports. The analysis is done in the context of fieldwork conducted in the USA and observations of experimental work at Glasgow University.