Ametistus gemma preciosa coloris violacei.
coloris ABCH e | colore f
preciosa (pre- e) ACH ef | pretiosa B
Ametistus is a violet-coloured precious stone.
ἀμέθυστος /améthystos/ in ancient Greek means literally "not drunken", also "not intoxicating", and it is said that this name reflects the perceived medicinal properties of the stone, i.e. preventing inebriation, a common belief in antiquity, which was also embraced by the Arabs and medieval Europe. Presumably this is why drinking vessels were made of this material. The word was adopted into Latin as amethystus. Simon's form ametistus reflects the medieval scribal habit of treating 'th' and 't', also 'y' and 'i' as interchangeable.
As a man of the clergy Simon was aware of the role that amethyst played in the Old Testament, Exodus, 28, 15ff, the pectoral of judgement, i.e. "the oracular breastplate of the Jewish high-priest" (Lewis & Short, 1879) is described. This breastplate was to be decorated with 12 gemstones in 4 rows of three, and the amethyst - Hebrew אחלמה /ʔaḫlā'māh/, Lxx ἀμέθυστος /améthystos/, Vulgate amethistus – was the last in the third row.
The amethyst is a variety of quartz []. Due to very extensive finds in the last two centuries amethyst has become a stone of less value, but in antiquity it was highly valued as a relatively rare gemstone. But even as early as in Simon's days it was no longer seen as rare.