A nugget is a lump of gold, or other precious substance, as found in the earth. It can also mean anything else which is valuable for its size, particularly something abstract such as a useful nugget of information.
As I lack both lumps of gold and the skill to photograph abstract concepts, here's a picture of Hev-Ock. She may be small, but I treasure her. My oldest (yet still incredibly youthful) friend gave her to me for luck and since then she's been a little nugget of inspiration.