This is from Ringing Church Bells to ward of Thunderstorms, a collection from Notes & Queries:
David de Pomis warned n 1587
There is very little of he true horn to be found most of that which is sold as being either stag’s horn or elephant’s tusk. The common test, which consists in placing the object in water to see whether bubbles will rise, is not at all to be trusted, and therefore wishing to benefit the world and to expose the wicked persons who sell worthless things at great prices, I take this occasion to describe a true test by which one may know the genuine horn from the false. the test is this: place the horn in a vessel of any sort of material you like and with it 3 or 4 live scorpions, keeping he vessel covered. If you find 4 hours later that the scorpions are dead, or almost lifeless, the unicorn’s horn is a good one, and there is not money enough in the world to pay for it.
The idea that all unicorn horns might be fraudulent gained credit very slowly. As early as 1527 the Scottish historian Hector Boethius had revealed that walrus tusks were straightened before being sold in Europe as unicorn horns, while the French traveller Andre Thevet writing in 1575, claimed actually to have seen Levantine artisans straightening tusks n an island in the Ed Sea preparatory to shipping them on as part of a lucrative trade. However doubt only spread after 1638, when Ole Wurm, a distignuished Danish professor, pronounced in Copenhagen that most of the celebrated ‘horns’ of Europe were not horns at all, but the tusks of narwahls. Values fell only modestly at first, but over the following decades a rising spirit of enquiry combined with a glut of horns combined to produce a dramatic collapse. By 1734 Johann Zedler’s great encyclopedia, the Universal-Lexicon, would report that horns that had previously fetched thousands of dollars could ow be bought for 25.