Rough volcanic rocks (kasei-gan) are usually used to represent mountains or as stepping stones. Smooth and round sedimentary rocks (suisei-gan) are used around lakes or as stepping stones. Hard metamorphic rocks are usually placed by waterfalls or streams. Rocks are traditionally classified as tall vertical, low vertical, arching, reclining, or flat. Rocks should vary in size and color but from each other, but not have bright colors, which would lack subtlety. Rocks with strata or veins should have the veins all going in the same direction, and the rocks should all be firmly planted in the earth, giving an appearance of firmness and permanence. Rocks are arranged in careful compositions of two, three, five or seven rocks, with three being the most common. In a three-arrangement, a tallest rock usually represents heaven, the shortest rock is the earth, and the medium-sized rock is humanity, the bridge between heaven and earth. Sometimes one or more rocks, called "suteishi" ("nameless" or "discarded"), are placed in seemingly random locations in the garden, to suggest spontaneity, though their placement is carefully chosen.