Hundreds of these 1-3 mm "seeds" have been found a couple of times this summer on my deck shaded by a large oak tree. But, they are not seeds with typical seed parts. Too small to be pollen or spores. I am baffled. They each have one or more deep crevices or slits longitudinally. Structurally, they appear to be composed of tiny (almost microscopic) wood chips. (Though I am sure this is just the appearance they have.) The first time I found them was after a rain and they had absorbed water and swelled to twice or three times their size and had become gooey and almost gelatinous to the touch. Almost looked like hundreds of little piles of brown poop on our deck. This time the weather was dry and they look as pictured in the photo. I swept up hundreds in a four-foot diameter area. Another area of the deck had a similar amount. None on the rest of our deck or any noticed in other areas under the oak tree. My biologist friend was also baffled. Please help identify
All right, I'll say it if nobody else will. Insect feces. The "wood chips" you saw are bits of your oak (or some other over-hanging tree) that have graduated from the arthropod digestive tract, so to speak. The pellets soak up water because the bugs are on a tight water budget and extract all the water before making a deposit. Never heard of a gall infestation that produced feces like this, but who knows? Look for dieing, deformed, or discolored leaves. Look for bugs on the tree and on fallen leaves. On the 2nd page of the document linked below is a larger specimen of what you are getting. http://collier.ifas.ufl.edu/CommHort/CommHortPubs/Stained_sidewalks_under_Oaks.pdf
Since these pellets are found under oak trees, I researched further and found a possible answer on the internet in a different location. Apparently, they are "galls" caused by insects disturbing the leaf structure of the oak trees. https://ask.extension.org/questions/157218 Not sure, still, what exactly is raining down in the pellets. Is it just dead plant material or is it something reproductive for the insects? I am very familiar with the ping pong ball "galls" that sometimes fall from these trees. Once they get wet and then dry again, they eventually become very powdery and crumble into dust.