Wood reaches jewelry in three main forms. Petrified wood is fossil wood whose tissue has been replaced by silica, making it as hard as quartz. Jet is a compact, black fossil wood from places such as Whitby in England. Bog oak is ancient oak preserved and darkened in peat bogs, carved into jewelry in Ireland. Each forms in a different way and needs different care.
How petrified wood forms
When a tree falls and is buried quickly by sediment or volcanic ash, it is cut off from oxygen and decays slowly. Groundwater rich in dissolved silica seeps into the wood. Over long periods, silica fills the cell spaces and then replaces the cell walls themselves. The result is a stone copy of the wood, often detailed enough to show growth rings, bark and cell structure.
Most petrified wood is made of chalcedony, a fine-grained form of quartz, with some opal or crystalline quartz. Iron oxides give reds, yellows and browns, manganese gives black and purple, and other trace elements add more colors. Opalized wood, from Australia, Nevada and other places, has opal as its main replacing mineral and may show play of color in rare cases.
Petrified wood rates about 6.5 to 7 on the Mohs hardness scale, far harder than other organic gems. It polishes to a glassy luster and is durable enough for rings, pendants and beads. The page on sedimentary rocks and gemstones explains the burial settings, and chalcedony covers the mineral that most often replaces the wood.
Arizona's Petrified Forest
Petrified Forest National Park in northeastern Arizona holds one of the largest concentrations of petrified wood in the world. The logs are about 225 million years old, from the Late Triassic, and lie in the Chinle Formation. Most came from extinct conifer trees that grew in a tropical river system and were buried by sediment rich in volcanic ash.
The area became a national monument in 1906 and a national park in 1962. Collecting petrified wood, or any rock, is not allowed in the park. Visitors who want petrified wood can buy pieces collected legally from private land outside the park, where shops in nearby towns sell it.
Rules for other public lands differ. On some US Bureau of Land Management land, hobby collectors may take small amounts of petrified wood for personal use under set limits. Commercial collection and collection in national parks are prohibited. Anyone collecting should check the current rules for the specific land.
Other petrified wood sources
Petrified wood is found worldwide. Sources include Madagascar, Indonesia, Egypt, Argentina, and several US states besides Arizona, such as Washington, Oregon, Wyoming and Utah. Washington named petrified wood its state gem in 1975. Indonesian petrified wood is often brightly colored and cut into large slabs and beads. Fossil palm wood from Texas and Louisiana shows rod-like fibers, and Texas named petrified palmwood its state stone in 1969.
Petrified wood is related to other fossils used in jewelry, such as agatized coral and dinosaur bone, covered on the page about fossils in jewelry.
Jet
Jet is a hard, compact form of lignite, a low-grade coal formed from wood. Driftwood from trees related to the modern monkey puzzle (Araucaria) sank into marine mud, where pressure and chemical changes over millions of years turned it into a dense black material. Jet is light, warm to the touch, and takes a high polish. It rates about 2.5 to 4 on the Mohs scale, with a specific gravity around 1.3.
The most famous jet comes from Whitby on the Yorkshire coast of England. Whitby jet formed in Early Jurassic rocks, about 180 million years ago. It washes out of the cliffs and is also mined from seams in the shale. People in Britain used jet for beads and ornaments as far back as the Bronze Age, and the Romans carved it in York.
Jet became the standard material for mourning jewelry in the 19th century. After Prince Albert died in 1861, Queen Victoria wore mourning dress for the rest of her life, and the court and much of the public followed. Whitby workshops expanded quickly to meet demand, producing carved brooches, lockets, bracelets and beads. The page on Victorian jewelry covers mourning jewelry in more detail. The trade declined as fashions changed in the early 20th century.
Spain, especially Asturias, and Turkey also produce jet. Spanish jet was carved into pilgrim souvenirs at Santiago de Compostela.
Jet imitations
Many black materials imitate jet:
- Black glass, often called French jet, is heavier, colder to the touch and harder.
- Vulcanite, a hardened rubber, can smell of sulfur when rubbed or warmed and may fade to brown.
- Bog oak is brown-black and shows wood grain.
- Black onyx, usually dyed chalcedony, is much harder and colder, and the page on onyx describes it.
- Plastics such as Bakelite are common in costume jewelry.
Jet leaves a brown streak on unglazed porcelain, while black glass leaves none. This streak test can scratch the piece, so test only a hidden spot or ask a gemologist.
Bog oak
Bog oak is wood from ancient oak trees that fell into peat bogs and was preserved for thousands of years. Acid, low-oxygen conditions slowed decay, while tannins in the wood reacted with iron in the water and turned it dark brown to black. Bog oak is partly fossilized but keeps much of its wood structure.
In 19th-century Ireland, carvers made bog oak jewelry and souvenirs with Irish motifs such as harps, shamrocks, round towers and castles. These pieces were sold to tourists and also worn as a less costly alternative to jet in mourning. Bog oak is softer and lighter than jet and shows visible wood grain.
Wood in modern jewelry
Contemporary jewelers also use ordinary hardwoods, burl, and stabilized wood. Stabilization fills the wood with resin under vacuum, which makes it harder and more water resistant. The page on contemporary and studio jewelry describes how such materials appear in studio work.
Care
Petrified wood is durable and can be cleaned with warm soapy water. Jet and bog oak are soft and can crack with sudden temperature changes or long soaking. Wipe them with a soft, damp cloth and dry them right away. Keep all of these materials out of ultrasonic and steam cleaners. The guide to cleaning jewelry safely gives more detail.
Frequently asked questions
Can you take petrified wood from Petrified Forest National Park?
No. Collecting petrified wood or any rock in the park is prohibited. Legally collected pieces from private land are sold in nearby towns.
Is jet a gemstone?
Jet is an organic gem. It is a compact fossil wood related to lignite and has been carved into jewelry since prehistoric times.
Why was jet used for mourning jewelry?
Its deep black color suited mourning dress. Demand rose sharply after Prince Albert's death in 1861, when Queen Victoria and her court adopted mourning fashions.
What is bog oak?
It is ancient oak preserved in peat bogs and darkened by tannins and iron. Irish carvers made jewelry and souvenirs from it in the 19th century.