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 through a single winter.
As any coastal dweller will observe, weathering is significantly accelerated by salt water. The illustration here shows a very crude experiment with a small fragment chipped from a pebble (not the same type of rock as the Runestone, but conveniently dark-coloured). This was placed in an orange-squash bottle cap (as at "Start") then had a little salt water dribbled on it. To simulate 19th century Minnesota climate, it was placed in a domestic food-freezer overnight, then taken out the next morning. The cap was immediately filled with very hot fresh water, which was then carefully drained before a photo was taken to show the particles left in the cap. The cap and fragment were rinsed with cool fresh water to remove all the particles (not wiped or rubbed- no abrasive techniques were used at any time) then left at room temperature all day. In the evening, the cycle began again with salt water and freezing.
The particles seen in the daily photos here therefore represent fresh weathering each day; notice that they tend to be very small in size, so the effects of the process, if continued over scores of freeze-thaw cycles throughout an entire winter (possibly even more than one cycle on really cold days), would be most noticeable at microscopic scales- exactly the way the Runestone was examined to establish the amount of weathering.
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...