Ancient magic on the sub? That's my jam!

It will probably be useful for us all to have the full quote, just so we know what we're looking at. Here it is, from book 36 of the Natural History:

‘Amiantus,’ which looks like alum, is quite indestructible by fire. It affords protection against all spells (veneficiis), especially those of the Magi.

I've used the Loeb translation, which is a little different from yours, and in fact the main difference gets us right into the meat of your question: what to do with the Latin word veneficium. If you look it up in a Latin dictionary (like this one), or do a quick search in the rest of Pliny's book (here), you'll see it can mean a whole range of different things: poison, potion, herb, curse, spell... So, what does it mean? Well, as is hopefully already becoming clear, the Romans used veneficium as a bit of a catch-all term for anything that was mysterious or unexplained and, importantly, could be used to harm another person. They didn't have the same scientific knowledge, nor the same standards of rationalisation as we do, so they made no distinction between what we call poisons (things that do harm because of their natural properties) and potions (concoctions that have a more supernatural or magical quality), meaning that they both got lumped together under the same terms; Pliny uses veneficium here but we also find venenum and the Greek pharmakon, which had a similar semantic range. There is a Roman law, passed in 81 BC, that deals with sicariis et veneficiis - assassins and poisoners - grouping the two together because they killed by stealth. However, by Pliny's time (late 1st century AD) the magical sense of the word had certainly grown, and Pliny's association of veneficium with the Magi is a dead give-away that he's not talking about what we would call 'poisons.' In the sense that he uses it here, I think Pliny is being purposefully vague about exactly what he means. The activities of the Magi were supposed to be mysterious and strange, so good, moral Romans weren’t supposed to know everything they could do. Elsewhere in Pliny he talks about them possessing arcane lore about unusual effects of plants, animals and stones. They can cure illnesses (e.g. at 28.260 where he says the Magi claim that insomnia can be cured by smearing nanny-goat bile on the eyes or under the pillow) or they can use plants to make love potions (27.57). Asbestos is not the only substance that Pliny claims can be used to defend against the activities of the Magi: he also extols the virtue of various plants (25.127) and even tortoise flesh (32.22) for counteracting particular veneficia or venena. In the case of the quote from your question, it comes from a long list of properties of various stones. As Pliny doesn’t say exactly what you’re supposed to do with the asbestos/amiantus, I suspect he’s thinking about its use as an amulet – perhaps worn on a string or just carried about the person in a pocket or pouch.

Hope this answers your question!