Spencer is an incorporated hamlet in Clark County, Idaho, United States. It serves as the base for the state's opal mine and is called the "Opal C…Spencer is an incorporated hamlet in Clark County, Idaho, United States. It serves as the base for the state's opal mine and is called the "Opal Capital of America": the mine proper is located 5–6 miles outside of town. The population was 37 at the 2010 census.