vein
A sheet-like filling of minerals occupying a crack in older rock, like a seam of material running through a block of stone.
Technically: In geology, a vein is a tabular body of minerals deposited by fluids that have crystallised within a fracture or fault in the host rock, typically under hydrothermal conditions. Veins are distinguished from dykes by their mineral (not igneous) fill, and from sedimentary layers by their cross-cutting or discordant relationship to surrounding strata. In economic geology, the term carries the specific implication that the mineralised fill is spatially discrete and bounded, which has direct consequences for how tonnage and grade are estimated during resource definition.
Geology