One of the first things that was noticed whan the Kensington Runestone was unearthed was that the tree roots across it were slightly flattened where they contacted the stone surface. This was a clear indication that they had been growing in contact with the stone for years, so it cannot easily be argued that Ohman (who had only lived in the area for a short time) was responsible for any supposed forgery. Recent scientific tests on the stone have demonstrated surface degradation comparable to that found in memorial stones which were carved centuries ago- but there are a couple of problems with such tests. The first is that the true surface of the stone as found was effectively cleaned off in the 1950s, when it was treated with oil and ether, which between them would remove the last traces of organic deposits from its burial. The second is that when testing the stone's surface it is difficult to know what assumptions to make about the significance of the results. Certainly, the carved surfaces are much more degraded than one would expect if they had been done relatively recently- but a forger would have had to make some attempt to degrade them artificially, because the stone contains mica, which twinkles like Xmas lights on any recently-exposed surface. That leads to the question of what methods could have been used to achieve this in 19th-century Minnesota. Chemicals could presumably be purchased by mail-order, but maybe they wouldn't be necessary. Even today after years of climate warming, Minnesota is well supplied with the most effective known means to degrade a rock surface- frost. It would be easy to simulate years of natural frost action by warming the stone each day, then leaving it out in the cold each night (sprinkled with water, or perhaps even coated with moistened clay) through a single winter. A really imaginative faker might also try making some chemicals- for example, you could probably get quite an interesting acid by squeezing the liquid out of a quantity of peat, and running it through a still. But all that is after the actual creation of the stone, including some runes and word-forms which appear to have been used only in a small area of Sweden in the Middle Ages, on documents which in some cases were not rediscovered by scholars until after 1898. Here's where the Document Challenge featured on my home-page comes into the picture...