Black sand is sand that is black in color. It seems to be very simple. But what is behind this concept? How is this type of sand formed? What is it made of? There is no single and easy answer to these questions because there are a number of different dark sand grains that can form black sand and hence there are several different ways how black sand can form.
The realm of black sands can be broadly divided into two parts, both of them having subdivisions. The most widespread type of black sand is composed of volcanic minerals and lava fragments. Such sands are especially common on the coasts of volcanic islands (Hawai’i, the Canary Islands, the Aleutians, etc.).
Black sand beaches are black because many volcanic minerals and rocks are dark-colored. Common rock types of volcanic islands are basalt (black when fresh), andesite (usually dark gray) and volcanic glass (often black in color). The minerals that give black color to these rocks are predominantly pyroxenes (mostly augite), amphiboles (mostly hornblende) and iron oxides (mostly magnetite). Such sands are heavier than ‘normal’ light-colored sands and become very hot on a sunny day. Dark color and heavyness are both caused by high iron content. Iron gives black color to most minerals because it absorbs light very well and it is also heavy.
Black volcanic sands may contain many non-black grains like green olivine crystals, reddish (usually because of weathering) volcanic rocks, light-colored quartz (when the source area is continental) and carbonate biogenic grains (coral sand). Most volcanic minerals are not very stable. They decompose pretty rapidly. These sands are said to be compositionally immature (mature sands are composed of quartz and other minerals very resistant to weathering). They also contain unusally high content of lithic (rock) fragments which have not broken up yet to form a sand composed of individual mineral grains.