Could be artificial. Some potting mixes (especially as used for hanging baskets) are made with water-retention granules so you don't need to water pots so often. They swell up like jelly and almost disappear as they dry out (giving the moisture back to the soil). If exposed to heavy rain, you would expect the smaller organic material to wash off them and make the jelly more obvious than usual.

Paul is right. The material is a hydrogel that is made from sodium acrylate, potassium acrylate, or an alkyl acrylate. This is the same stuff that is found inside disposable diapers (nappies). Similar materials are used in kids toys, the small animals that you put in water that grow to huge proportions and then shrink back when they dry out.

There are quite a few materials that behave like this. For instance, try putting a jelly baby or gummie bear in water overnight.