Northwest corner of Highway 49 and McHatten Alley in the city of Diamond Springs. It's the intersection of Pleasant Valley and China Garden Roads.
From Discover Historical California:
Diamond Springs was one of the richest gold areas around Placerville. A 25-pound gold nugget, among the largest found in the county, was discovered here in 1850. Several restored buildings remain in this small town including the 1852 I.O.O.F. (Independent Order of Odd Fellows?) Hall.
From the official site plaque:
This town, settled in 1848, derived its name from its crystal clear springs. Among the richest spots in this vicinity, its diggings produced a 25 pound nugget, one of the largest ever found in El Dorado County. Its most thriving period was in 1851. Later it became a contender for the county seat. Through its lumber, lime production and agriculture, Diamond Springs retained some of its early importance.
California Registered historical landmark number 487. Plaque placed by the State Department of Parks and Recreation in cooperation with the citizens of Diamond Springs and the Mother Lode Lion's Club of Diamond Springs and El Dorado.