“There are tons of free and paid icon collections, vector and pixel, 2D and 3D, and if you want to write rules on how to deal with them all you’d need to write a big icon cookbook. There are so many damn icons that the devil might trick you into solving all of the world’s design problems with them.”
Just as typography, keeping a consistent icon set within your app is a major challenge. The Noun Project creators come lend a helping hand “creating, sharing and celebrating the world’s visual language”.
“Material Design Icons’ growing icon collection allows designers and developers targeting various platforms to download icons in the format, color and size they need for any project.” You can contribute too!