The Spinelli prospect, located in Glastonbury, Hartford County, Connecticut, USA, was developed at the beginning of the 20th century on a small pegmatite outcrop, most likely for small-scale feldspar and mica mining. Its significance among many other Connecticut pegmatites was augmented by the pivotal role samarskite from the prospect played in the development of U-Pb dating and by the use of other minerals from the same location in a variety of other radioisotopic dating measurements. Old documents, some never published, reveal that in the early days of radioisotopic dating it was Wilbur Foye of Wesleyan University who made the Spinelli prospect samarskite available to scientists at Harvard University, and from there the US Geological Survey and National Research Councils Committee on the Measurement of Geologic Time. Very little has been published about the detailed mineralogy of the host pegmatite and the chemistry of Spinelli samarskite, and almost nothing about the prospects ownership history. The present paper fills these gaps and includes historical aspects, an updated map, a radiological survey of samarskite distribution in the outcrop, the paragenesis of the Spinelli prospect pegmatite and the geochemistry of its most famous mineral.