u/Electrical-Smile-636

There was sticky dark goo on the soles of these shoes. My daughter doesnt remember where this happened, but it has some small stones in it, so I expect it is some mud and maybe resin as it is sooo sticky.

I tried removing it by scraping, rubbing with iron wool, detergent, Biotex, green soap. The thick parts are mostly gone, but the dark stain remains. Any tips on how to get rid of this?

Help much appreciated!