Peridot (/ˈpɛrɪdɒt/ or /ˈpɛrɪdoʊ/) (sometimes called chrysolite) is gem-quality olivine and a silicate mineral with the formula of (Mg, Fe)2SiO4. As peridot is a magnesium-rich variety of olivine (forsterite), the formula approaches Mg2SiO4. Its green color is dependent on the iron contents within the structure of the gem. Peridot occurs in silica-deficient rocks such a volcanic basalt as well as in pallasitic meteorites. Peridot is one of only two gems observed to be formed not in the Earth’s crust, but in molten rock of the upper mantle. Gem-quality peridot is rare to find on Earth's surface due to its susceptibility to weathering during transportation from deep within the mantle to the surface.

In the Middle Ages, the gemstone was considered a stone that could provide healing powers, curing depression and opening the heart. Peridot is the birthstone for the month of August and the 16 year wedding anniversary gemstone.

The origin of the name peridot is uncertain. The Oxford English Dictionary suggests an alteration of Anglo–Norman pedoretés (classical Latin pæderot-), a kind of opal, rather than the Arabic word faridat, meaning "gem".

The Middle English Dictionary's entry on peridot includes several variations: peridod, peritot, pelidod and pilidod — other variants substitute y for the is seen here.

The earliest use in England is in the register of the St Albans Abbey, in Latin, and its translation in 1705 is possibly the first use of peridot in English. It records that on his death in 1245, Bishop John bequeathed various items, including peridot, to the Abbey.

Peridot is one of the few gemstones that occur in only one color: an olive-green. The intensity and tint of the green, however, depends on the percentage of iron in the crystal structure, so the color of individual peridot gems can vary from yellow, to olive, to brownish-green. In rare cases, peridot may have a medium-dark toned, pure green with no secondary yellow hue or brown mask.

Refractive index readings of faceted gems contain indices α = 1.653, β = 1.670, and γ = 1.689 with a corresponding bi-axial birefringence of 0.036. Study of Chinese peridot gem samples determined the specific gravity hydro-statically to be 3.36. The visible-light spectroscopy of the same Chinese peridot samples showed light bands between 493.0 and 481.0 nm, the strongest absorption at 492.0 nm.

Inclusions are common in peridot crystals but the variety depend on the location it is found at. Stones from Pakistan contain silk and rod like inclusions as well as black chromite crystal inclusions surrounded bi-circular cleavage discs resembling lily pads, and finger print inclusions. Brown Mica flakes are more evident in Arizona gems.

The largest cut peridot olivine is a 310 carat (62 g) specimen in the Smithsonian Museum in Washington, D.C.

Peridot has been prized since the earliest civilizations for its protective powers to drive away fears and nightmares. It's believed to carry the gift of inner radiance, sharpening the mind and opening it to new levels of awareness and growth, helping one to recognize and realize one’s destiny and spiritual purpose.

Ancient Egyptians believed that Peridot was sent to Earth by the explosion of a star and carried its healing powers. Peridot is the national gem of Egypt known to locals as the Gem of the Sun.

Peridot is sometimes mistaken for emeralds and other green gems. Notable gemologist George Frederick Kunz discussed the confusion between emeralds and peridot in many church treasures, notably the "Three Magi" treasure in the Dom of Cologne, Germany.

Peridot olivine is the birthstone for the month of August and the 16th wedding anniversary gemstone.