The Assay Office is a historic building at 210 Main Street in Boise, Idaho. It is significant for its role in the history of mining in Idaho, and was the first major federal government building in the Idaho Territory. During the first half of the 1860s, Idaho's gold production was the third highest in the nation. Due to the difficulty of transporting bulky, heavy ores the long distance to the nearest U.S. Mint in San Francisco, there was great demand for an assaying office in Idaho. Gold and other precious metals are not mined in a pure form. In order to place a value on an ore, the precious metal must be separated from the impurities. This is what an assay office does.