The Silver Nugget is the result of putting a Silver Ingot into the crafting bench (Redpower), if Thaumcraft is installed it can be used for transmutation as long as you have researched it. To transmute silver nuggets, Silver Transmutation must have been researched. It is nearby Basic Alchemy and Gold Transmutation in the Thaumonomicon, and those are prerequisites for research.
Silver Ingots can be crafted using 9 silver nuggets and are used as a replacement for redstone in a more efficient recipe for Glass Fibre Cable in IndustrialCraft 3 and IndustrialCraft 2, and to make Blue Alloy Ingot and Athame in RedPower 2.
Silver Nugget Texture