A quartz vein cutting through a limestone boulder.
A vein is a mineral formation of far greater extent in two dimensions than in the third.
Sheetlike fissure veins occur where fissures formed in the rock become filled with mineral. Ladder veins formed in series of fractures in, for example, dikes. Saddle veins are lens-shaped, concave below and convex above. Veins that contain economically important ores are often termed lodes.