Spencer is a city in Clark County, Idaho, United States. The population was 38 at the 2000 census. The city is noted as being the "Opal Capital of America". Travellers passing through Spencer along I-15 may stop to purchase rough or polished opal at one of the many local shops. The mine is located 5–6 miles outside of town. Spencer opal is polished into cabochons known as singlets, doublets or triplets. Singlets are the most prized of the Spencer cabs, because they are the most difficult to create as the color usually shows best when backings are used . Usually, finished Spencer opal will come with one backing (doublet) or with a backing and quartz dome (triplet). Finished Spencer opal are among the most spectactular opals in the world, rivaling the best Australian opal.