Beryl (/ˈbɛrəl/ BERR-əl) is a mineral composed of beryllium aluminium silicate with the chemical formula Be₃Al₂(SiO₃)₆. Well-known varieties of beryl include emerald and aquamarine. Naturally occurring hexagonal crystals of beryl can be up to several meters in size, but terminated crystals are relatively rare. Pure beryl is colorless, but it is frequently tinted by impurities; possible colors are green, blue, yellow, pink, and red (the rarest). It is an ore source of beryllium.

The word beryl – Middle English: beril – is borrowed, via Old French: beryl and Latin: beryllus, from Ancient Greek βήρυλλος bḗryllos, which referred to various blue-green stones, from Prakrit veruḷiya, veḷuriya 'beryl' which is ultimately of Dravidian origin, maybe from the name of Belur or Velur, a town in Karnataka, southern India. The term was later adopted for the mineral beryl more exclusively.

When the first eyeglasses were constructed in 13th-century Italy, the lenses were made of beryl (or of rock crystal) as glass could not be made clear enough. Consequently, glasses were named Brille in German (bril in Dutch and briller in Danish).

Beryl is a common mineral, and it is widely distributed in nature. It is found most commonly in granitic pegmatites, but also occurs in mica schists, such as those of the Ural Mountains, and in limestone in Colombia. It is less common in ordinary granite and is only infrequently found in nepheline syenite. Beryl is often associated with tin and tungsten ore bodies formed as high-temperature hydrothermal veins. In granitic pegmatites, beryl is found in association with quartz, potassium feldspar, albite, muscovite, biotite, and tourmaline. Beryl is sometimes found in metasomatic contacts of igneous intrusions with gneiss, schist, or carbonate rocks. Common beryl, mined as beryllium ore, is found in small deposits in many countries, but the main producers are Russia, Brazil, and the United States.

New England's pegmatites have produced some of the largest beryls found, including one massive crystal from the Bumpus Quarry in Albany, Maine with dimensions 5.5 by 1.2 m (18.0 by 3.9 ft) with a mass of around 18 tonnes (20 short tons); it is New Hampshire's state mineral. As of 1999^[update], the world's largest known naturally occurring crystal of any mineral is a crystal of beryl from Malakialina, Madagascar, 18 m (59 ft) long and 3.5 m (11 ft) in diameter, and weighing 380,000 kg (840,000 lb).

Beryl belongs to the hexagonal crystal system. Normally beryl forms hexagonal columns but can also occur in massive habits. As a cyclosilicate beryl incorporates rings of silicate tetrahedra of SiO₃–O (the connected O is from another SiO₃) that are arranged in columns along the C axis and as parallel layers perpendicular to the C axis, forming channels along the C axis. These channels permit a variety of ions, neutral atoms, and molecules to be incorporated into the crystal thus disrupting the overall charge of the crystal permitting further substitutions in aluminium, silicon, and beryllium sites in the crystal structure. These impurities give rise to the variety of colors of beryl that can be found. Increasing alkali content within the silicate ring channels causes increases to the refractive indices and birefringence.

Beryl is a beryllium compound that is a known carcinogen with acute toxic effects leading to pneumonitis when inhaled. Care must thus be used when mining, handling, and refining these gems.

Aquamarine (from Latin: aqua marina, "sea water") is a blue or cyan variety of beryl. It occurs at most localities which yield ordinary beryl. The gem-gravel placer deposits of Sri Lanka contain aquamarine. Green-yellow beryl, such as that occurring in Brazil, is sometimes called chrysolite aquamarine. The deep blue version of aquamarine is called maxixe (pronounced mah-she-she). Its color results from a radiation-induced color center.