LuxRender has started a Materials Database from which you can download materials as well as upload you own content to it. It looks pretty impressive so far. While you are at the LuxRender site check out the gallery too. There are some wonderful Blender LuxRenders in the exhibit.
I step by step breakdown of my way of achieving a pearl material with luxrender. It describes and highlights some aspects of the luxrender material system.
Close to 60 different ‘organic’ materials.
45+ nature type materials.
An library of odd and ends: stuff like electric bolts, baseball, cigarette filter and lollipop.
A large collection of Metal materials.
A collection of Liquids materials.
A collection of ‘halo’ materials.
Close to 30 different glass type materials.
Close to 20 ‘fur’ materials including grass, hair and wool.