The Europeans have encountered for the first time what are now called diamonds during the expedition of Alexander the Great in India, where they had been appreciated for jewelry for a long time.
This is why the Romans, e.g. Pliny the Elder, used for diamonds the name "Indian adamant". The name "adamant" without the "Indian" specifier had already been used for many hundreds of years (the oldest attestation is in Hesiod), but it had not been applied to a gem, but to nuggets of native osmium-iridium alloy, which can be found mixed with the nuggets of native gold in the alluvial deposits of gold (and which, unlike the gold with which they were mixed, could be neither melted nor forged, hence their name, "untameable"; "unconquerable", which is used in the article is a bad translation for "adamant").
Pliny the Elder described the "Indian adamant" as consisting of octahedral crystals, which is the most frequent form of the natural diamonds. It appears that at that time it was impossible to cut the diamonds, at most they might have been polished a little, so the crystals used in jewelry retained their native shape.
By the time of Pliny the Elder, the "Indian adamant" was the most expensive gem, surpassing even the noble opals, the pearls, the emeralds and the beryls, which were the next most expensive gems at that time. The Romans did not care much for transparent crystals, they appreciated much more the higher quality exemplars of noble opals or pearls, if those exhibited a nice play of colors.