You can save your layers as Smart Materials if you want to use or save them outside of a project.See : - https://support.allegorithmic.com/documentation/display/SPDOC/Smart+Materials- https://www.youtube.com/watch?v=B3oruU-cgUU
If you imported them in the project rather than placing them into your shelf, then the only way to "get them back" is by creating a smart material. You can't extract a substance otherwise.