August Birthstone

August carries three recognized birthstones — peridot and spinel as modern options, sardonyx as the traditional one — a wider spread than most months, and one that includes what's arguably the most geologically unusual gem on the entire birthstone calendar.

Peridot: A Gem From the Mantle (and Sometimes Space)

Peridot is the gem-quality variety of olivine, and it's genuinely unusual among gemstones in that it forms deep in the Earth's upper mantle rather than in the crust, reaching the surface through volcanic activity that carries mantle material upward. It's also one of the very few gemstones that has been found in meteorites — specifically in a rare meteorite type called pallasites, which contain visible peridot crystals embedded in a metallic matrix, offering scientists a literal sample of material from outside Earth. Its vivid lime-to-olive green color comes from iron within the crystal structure, and unlike many colored gems, peridot's color doesn't vary by trace-element differences the way ruby's or emerald's does — olivine's color range is comparatively narrow and consistent. Ancient Egyptians mined peridot on Zabargad Island (also called St. John's Island) in the Red Sea as far back as around 1500 BCE, and some historians believe a portion of what's remembered in antiquity as 'Cleopatra's emeralds' may actually have been peridot from this source. It's sometimes nicknamed the 'evening emerald' for a warm glow it can take on under artificial light.

Spinel: The Great Impostor

Spinel has one of the more remarkable backstories in gemology: for centuries, red spinel was mistaken for ruby, and it wasn't reliably distinguished as a separate mineral species until modern gemological testing became available. Several famous 'rubies' set in historic royal jewelry turned out, on closer examination, to actually be spinel — most famously the Black Prince's Ruby, a large red gem set in the British Imperial State Crown that has been definitively identified as spinel rather than true ruby. Spinel occurs in a wide range of colors beyond red, including vivid pink, blue, and violet, and in recent years it's gained recognition in its own right as a durable, richly colored gem rather than simply being thought of as ruby's cheaper cousin. It rates 8 on the Mohs scale, making it a solidly durable choice for everyday jewelry.

Sardonyx: An Ancient Seal Stone

Sardonyx is a banded variety of chalcedony (a form of quartz), combining layers of sard — a brownish-red variety — with layers of white or lighter-colored onyx, producing a distinctive striped appearance. Its flat, even bands made it a favored material for engraved seals and cameos in ancient Rome, where the contrast between layers allowed carvers to create a raised, contrasting design by cutting into one band to reveal the color underneath. It rates 6.5–7 on the Mohs scale.

A Month of Contrasts

Between a mantle-and-meteorite gem, a mineral historically mistaken for one of the world's most famous stones, and an ancient carving material, August's three birthstones span an unusually wide range of gemological stories for a single calendar month.

Peridot's Modern Sources and Hawaii's Green Sand

Beyond ancient Zabargad, modern peridot comes primarily from Arizona's San Carlos Apache Reservation, one of the largest peridot deposits in the world, along with Pakistan and China, both of which have produced exceptionally large, fine-quality crystals since the late 20th century. Peridot's mantle origin also explains a striking natural phenomenon: several beaches in Hawaii, most famously Papakōlea (Green Sand Beach) on the Big Island, get their distinctive green color from eroded olivine crystals washed out of volcanic deposits — genuine peridot-grade material scattered as sand rather than cut gemstones, a direct visible reminder of the mineral's deep volcanic origin.

Why the Black Prince's Ruby Mattered to Gemology

The Black Prince's Ruby is more than a curious footnote — its correction from 'ruby' to 'spinel' is frequently cited by gemologists as a landmark moment in the field's own history, illustrating how unreliable naming by color and appearance alone could be before rigorous mineralogical testing (measuring refractive index, specific gravity, and crystal structure) became standard practice. Multiple other historic 'rubies,' including several in the Russian Crown Jewels and elsewhere in European royal collections, have similarly been reclassified as spinel once modern gemological testing was applied, part of why spinel has shed much of its old reputation as merely a lesser substitute and gained real standing as a distinct, desirable gem in its own right, particularly in bright, saturated 'neon' pink and orange varieties from Burma and Tanzania that have become popular with contemporary designers.

Sardonyx Versus Plain Onyx and Sard

Sardonyx sits at the intersection of two related but distinct chalcedony varieties: sard, a translucent brownish-red to orange-brown stone, and onyx, typically black or banded black-and-white. When bands of sard and lighter onyx or chalcedony occur together in a single stone, the combined material is called sardonyx; when the stone is uniformly black without the reddish sard layers, it's simply called onyx, a separate August-adjacent stone historically associated more with mourning jewelry in Victorian tradition than with the sardonyx's ancient sealing and carving use described above.

Peridot in Ancient and Medieval Religious Objects

Beyond its Egyptian mining history, peridot has a documented presence in medieval European religious treasuries: several church reliquaries and ecclesiastical pieces long catalogued as containing emerald have, on closer modern gemological examination, turned out to actually contain peridot instead — a mix-up that historians attribute partly to poor artificial lighting in medieval settings, under which peridot's green can appear closer to emerald's than it does in daylight, and partly to peridot and emerald simply not being reliably distinguished by pre-modern gemological knowledge. The Three Kings Shrine at Cologne Cathedral in Germany is among the most frequently cited examples of this historical misidentification, containing green stones long assumed to be emerald that modern analysis identified as peridot.

How Spinel's Colors Form

Spinel's wide color range comes from a genuinely diverse set of trace elements substituting into its magnesium aluminum oxide crystal structure: chromium produces spinel's red and pink shades (the same element responsible for ruby's red), while iron and cobalt are associated with its blue and violet varieties, and different combinations produce the vivid orange and 'Jedi' purple-pink tones that have become particularly popular with contemporary jewelry designers over the past two decades. Because spinel crystallizes in the cubic system rather than corundum's trigonal system, spinel crystals typically form as octahedrons (eight-sided shapes), a distinctly different rough crystal habit from ruby and sapphire that experienced gem cutters can sometimes use to help identify rough material before it's even cut.