Materials are optional in Umbra. If you omit materials on an object, Umbra will use a placeholder material for that object.
Umbra assumes all materials to be instances of "Standard Material" (the default material in Unity). Materials with similar properties may work, but are not officially supported. Umbra will output new materials which are all instances of "Standard Material". There is currently no way for the user to define their own materials that get handled by Umbra.
Umbra currently only supports opaque materials. Transparency or translucency of a material is ignored.