August Birthstone: Peridot, Spinel, and Sardonyx
August Birthstone: Peridot, Spinel, and Sardonyx
A trio of distinct gemstones spanning ancient banded chalcedony to vivid olivine and the newly recognised spinel
August is one of only three months on the modern American birthstone list to carry three officially recognised stones: peridot, the long-established primary gem; spinel, added in 2016; and sardonyx, a historical designation that predates the standardised lists of the twentieth century. Each stone is mineralogically and optically distinct, and their coexistence on a single month's list reflects both the evolution of gemmological knowledge and the commercial pressures that have periodically reshaped birthstone conventions.
Peridot: The Traditional August Stone
Peridot is the gem-quality variety of the mineral olivine, a magnesium iron silicate with the chemical formula (Mg,Fe)2SiO4. Its colour — ranging from a yellowish lime green through a rich olive to a deeper bottle green — is caused entirely by the presence of iron within the crystal structure, making peridot one of the few gemstones that occurs in only a single colour family. It crystallises in the orthorhombic system, with a refractive index of approximately 1.654–1.690 and a birefringence of 0.035–0.038, the latter sometimes producing a characteristic doubling of back facets visible to the unaided eye in larger stones.
The association of peridot with August is ancient in spirit if not in formal codification. The island of Zabargad (St John's Island) in the Egyptian Red Sea was mined for peridot for at least two millennia, and stones from this source were known to the classical world under the name topazios — a source of considerable historical confusion with the mineral topaz. The modern standardised birthstone list, published by the (then) National Association of Jewelers in the United States in 1912, confirmed peridot as the stone for August, cementing an association that had existed in various earlier European lists.
Today, significant peridot sources include the San Carlos Apache Reservation in Arizona (which supplies a large proportion of the commercial market), Suppat in Pakistan's Kohistan district (yielding finer, larger crystals), Myanmar's Pyaung Gaung locality, and China's Hebei province. Peridot of extraterrestrial origin — recovered from pallasite meteorites — is scientifically documented and occasionally faceted, though it remains a curiosity rather than a commercial category.
Spinel: The 2016 Addition
In 2016, the American Gem Trade Association (AGTA) and Jewellers of America jointly announced the addition of spinel to the August birthstone list — the first revision to the list since tanzanite was added for December in 2002. Spinel is magnesium aluminium oxide (MgAl2O4), crystallising in the cubic system with a refractive index of approximately 1.718. It occurs in a wide range of colours, including vivid reds, hot pinks, oranges, blues, and near-colourless, with colour determined by trace elements such as chromium (red and pink), iron (blue and violet), and cobalt (vivid blue in rare cases).
The rationale for spinel's inclusion was twofold: to offer consumers greater choice beyond peridot's single colour range, and to acknowledge spinel's extraordinary historical significance — a stone that, for centuries, was confused with ruby and sapphire in royal treasuries across Asia and Europe. The Black Prince's Ruby in the British Imperial State Crown and the Timur Ruby in the Royal Collection are both, in fact, large red spinels. Notable spinel localities include Mogok in Myanmar, Tajikistan's Kuh-i-Lal deposit, and more recently the Mahenge plateau in Tanzania, source of the intensely fluorescent hot-pink spinels that attracted considerable market attention in the early 2000s.
Sardonyx: The Historical Alternative
Sardonyx is a layered variety of chalcedony — itself a cryptocrystalline form of quartz — in which brownish-red sard alternates with white or black onyx bands. It has been used for cameos and intaglios since antiquity, prized precisely because its contrasting layers allowed carvers to reveal a light image against a dark ground or vice versa. Sardonyx appears on several pre-1912 European birthstone lists for August, reflecting its long history as a gem of late summer.
In the modern commercial context, sardonyx is seldom actively promoted as an August birthstone. It does not appear on all contemporary lists, and where it does, it is typically noted as a historical or alternative option rather than a primary recommendation. Its relatively modest price point and the dominance of peridot in August marketing have marginalised it commercially, though it retains genuine interest among collectors of antique intaglios and cameo jewellery.
Comparative Summary
- Peridot — olivine; orthorhombic; RI 1.654–1.690; olive to lime green; primary August stone since 1912.
- Spinel — magnesium aluminium oxide; cubic; RI ~1.718; wide colour range; added to August list in 2016 by AGTA and Jewellers of America.
- Sardonyx — banded chalcedony (sard and onyx); cryptocrystalline quartz; brown-red and white or black layers; historical designation, seldom commercially promoted today.