I know that I've used this before to illustrate Marius .... but I should actually point out that I don't believe it's an image of him.
I have no idea where the 'original' is now, but it's meant to be vitreous paste engraved with his portrait and found at Palestrina.
Palestrina is ancient Praenesta, where his son died - hence this would be a very good 'provenance' for a collector, since it would suggest that it is pre 82 BC and was owned by his son or one of his supporters ... We tend to think of fake provenances as being a feature of the modern art market - to hide the origins of looted objects - but they were used from the Renaissance onwards to add 'interest' to objects.
Here's the problem. Marius was one of Rome's 'great' so all collectors wanted an image of him, and many catalogues of cameos and signets I've looked through claim to include images of him .... but there don't seem to have been any that survived, so these were faked.
The reason that I know that this is a fake, without having seen it, is that 'Marius' has a beard. The very early Republicans wore beards, but these went out of fashion and were not revived until Hadrian. In Marius' day one did not have a beard - and the only time he is recorded with a beard was when he was in mourning during his exile (ie in 88 & 87, before his the 7th consulship of 86 in the inscription). Mourning was practised when someone died, when one was accused of a serious crime or when in exile - family and supporters also wore mourning - but since this was considered a 'low' period in one's life, and one was depicted in portraits at the height of one's glory ... this cannot be a portrait of Marius.