Mineralpedia Details for Sylvanite
Named for the locality where it was first found in Transylvania (sylvan) in Romania and also for its tellurium content as an early name suggested for tellurium was sylvanium. Sylvanite occurs in hydrothermal veins of low temperature but can also be found as one of the last-formed minerals in medium-temperature to high-temperature deposits as well. It is present in small amounts in hundreds of gold-silver deposits, but not typically as rich specimens or as an economically viable mineral. Sylvanite is a light sensitive mineral and can tarnish darkly if exposed to light for long periods.
Ref. Handbook of Mineralogy, Anthony et al (1995) and MSA at http://www.handbookofmineralogy.org/pdfs/sylvanite.pdf
Sylvanite from Sacarîmb (Nagyag in Hungary), Romania
Arborescent crystal growth with individual crystals from 5 to 10mm.
Sylvanite from Portland mine, Lawrence Co., South Dakota, United States
Streaks and this sheets to 1cm.
Sylvanite from Cresson mine, Cripple Creek, Teller Co., Colorado, United States
Numerous brassy gold, striatated and tabular crystals of Sylvanite up to 4mm,
Sylvanite from Emperor mine, Tavua Gold Field, Viti Levu, Fiji
This is a very rich specimen with a few dozen or more sharply terminated, goldish prismatic Sylvanite crystals, the largest of which is 4mm.
Sylvanite from Spotted Horse mine, Fergus Co., Montana, United States
Here's a telluride locality, which you may not have heard of. The specimen is very rich with goldish platy Sylvanite 2-3mm wide.