the moon's reflection in the water. It is also mentioned in that cascades benefit from being located in such a manner that they are half-hidden in shadows. Rocks and sand Rock, sand and gravel are an essential feature of the Japanese garden. A vertical rock may represent Mount Horai, the legendary home of the Eight Immortals, or Mount Sumeru of Buddhist teaching, or a carp jumping from the water. A flat rock might represent the earth. Sand or gravel can represent a beach, or a flowing river. Rocks and water also symbolize yin and yang ( and in Japanese) in Buddhist philosophy; the hard rock and soft water complement each other, and water, though soft, can wear away rock. Rough volcanic rocks () are usually used to represent mountains or as stepping stones. Smooth and round sedimentary rocks () 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 ("nameless" or "discarded"), are placed in seemingly random locations in the garden, to suggest spontaneity, though their placement is carefully chosen. In ancient Japan, sand () and gravel () were used around Shinto shrines and Buddhist temples. Later it was used in the Japanese rock garden or Zen Buddhist gardens to represent water or clouds. White sand represented purity, but sand could also be gray, brown or bluish-black. Selection and subsequent placement of rocks was and still is a central concept in creating an aesthetically pleasing garden by the Japanese. During the Heian period, the concept of placing stones as symbolic representations of islands – whether physically existent or nonexistent – began to take hold, and can be seen in the Japanese word , which