Small stones. or gravel, like the stones one finds on the beach. usually 10 mm or 20 mm. The 20 mm are usual for walking on and the 10 mm, which are also called pea shingle, are used as a mulch in pots to stop evaporation on the surface.
Ok, there are three types of "shingles" I am thinking about when reading your questions. The first shingles are the tiles you put on the roof of a house. The second shingles are in your second question, single underfoot. That means there are stones, pebbles, little rocks, and just junk stuck on the bottom of your foot. It takes a while to dig those out. Third, the sickness "shingles". This type of shingles is a virus related to chicken-pox. The way you get this sickness is by having it dormat in your system for years when it suddenly flares. However, you can only have it if you have had chicken-pox before.