This is a major chalcocite, with robust, fat, impressive crystals that look for all the world like blacker hematite than chalcocite from Bristol, because they are so thick and large. This specimen has solid rock matrix, as the host for twinned clusters to 2.5 cm. From the noted rarities and classics collection of Thomas E. Egleston (1832-1900) Professor of Mineralogy and Metallurgy, Columbia University, New York (and retains his old label). This specimen is one of the prettier Bristol chalcocites in this size range I have seen, for the sharp aesthetics of distinctly twinned, upright crystals perched on such contrasting matrix. Commercial mining began in 1837 and carried on until 1953 (according to MINDAT.ORG) - this mine was a hugely important US resource through the Civil War years and beyond, and collectors lament its closing. Most regard the chalcocite from this mine as being the best on the planet, though English ones are pretty good too, I will say. Certainly, this is the most famous old locale in the US for the species and you can see why from an exemplary piece such as this.