1. Here's why
It is currently only possible to add Standard Materials and variations of the Standard Material from the built-in Material Library to the Favorites tab.
In other words, it is not possible to add the following materials:
- Indoor category:
- Outdoor category:
- Nature category:
- Custom category:
- Imported Material
Please also note that it is not possible to name or rename materials on the Favorites tab.
This is a technical constraint.
2.1: Save your materials as files and load them when needed:
Knowledge Base article: How do you copy, paste, save and load Materials?