You can drag any material from the scene to a folder and it will be saved as a re-usable file. However, it will not have embedded thumbnail, like the images in materials library. You can tell Owlet to generate such image and embed it into the material file. This is useful if you want to extend the standard materials library with your own, or share them with someone.
Here’s how you do it:
Owlet.exe --thumbnail material.owletMaterial scene.owletScene
It is similar on Mac OS:
Owlet.app/Contents/MacOS/Owlet --thumbnail material.owletMaterial scene.owletScene
The first parameter is the material file you want to update with thumbnail. The second is the scene to use for rendering a thumbnail. The scene needs to have a material named “Material” in order to be used for rendering thumbnails. You can use the scenes that come with Owlet. On Windows they are located in Preview folder next to Owlet.exe. On MacOS they are located inside the application bundle in Owlet.app/Contents/Resources/Preview.