Zinkenite is a metallic sulfosalt, a lead antimony sulfide. It often occurs as acicular needle-like crystals, and is one of only a few sulfide minerals that appear this way (jamesonite, boulangerite and millerite are other sulfides that form similar acicular crystals). Here, however, you have large, sizeable crystals, rather than just hair-like ones, and this is a very rich specimen for the species. The striated crystals have great luster. The crystals only at the very ends are not terminated - probably both ends touched the pocket walls, and this is typical. However the crystals you do see are really robust, unusually lustrous and long.