1. Anderson, B. W., and Jobbins, E. A. 1990, Gem Testing: 10th edn. Butterworths.
2. Anon, 1497, Hortus Sanitatis, De lapidibus et in terre venis nascentibus, medieval compilation, publ. in Mainz, author not known with any certainty, copy University of Madrid Library. [see chapter 109 on lapis, but does not add much to what is in the earlier Marbodus, Beckmann 1799]
3. Bandy, M. C., and Bandy, J. A. 2004, De natura fossilium, Dover Publs. [Agricola’s Mineralogy]
4. Burton, R. (Democritus Junior), 1628, The anatomy of melancholy: H. Cripps, Oxford.
5. Caley, E. R., and Richards, J. F. S. 1956, Theophrastus on stones, The Ohio State University.