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Opalized Wood

Author: , PhD, GIA Graduate Gemologist

Herringbone Sequoia

Herringbone Sequoia: These cabochons were cut from an opalized wood known as Herringbone Sequoia. The rough was found in the Snake River / Hell's Canyon area by an old-time rockhound in the mid-1900s and was sold as part of his estate. It is uncertain if it was found on the Idaho or Oregon side of the canyon. Whichever state it came from, it is a beautiful and unique material. It is definitely opalized wood (specific gravity = 2.106, spot refractive index = 1.48). These cabochons were cut by Greta Schneider of Copper Creek Cabs.


What Is Opalized Wood?

Opalized wood is a type of petrified wood that is composed of opal rather than chalcedony or another mineral material. It almost always consists of common opal, without play-of-color, but rare instances of petrified wood composed of precious opal are known.

Opalized Wood Cabochon

Opalized Wood: A cabochon made from opalized wood from eastern Oregon. This cabochon measures approximately 11.5 x 17 millimeters and weighs 5.35 carats. GIA Lab Report for this stone.

How Does Opalized Wood Form?

One of the most common and best geologic environments for the formation of petrified wood is a forest buried by a volcanic ashfall. In this situation the ash buries the plants and protects them from decay and insect attack. The ash also serves as an abundant source of easily dissolved silica, which will be carried into the wood by moving groundwater where it precipitates in cavities and replaces the solid woody materials. Large deposits of petrified wood in Arizona, Oregon, Wyoming, Indonesia, Russia, Mexico, Brazil, and other parts of the world have formed in this environment.

In most situations, the petrified wood found in these deposits today is composed of chalcedony, but in some situations the wood is composed of opal. Both of these varieties of petrified wood often occur in a single deposit. Because they are both formed from dissolved silica, they are often called "silicified wood".

Opalized Wood Tumbled Stone

Opalized Wood Tumbled Stone: A large tumbled stone made from opalized wood. This stone is about 2 inches across.


How To Identify Opalized Wood

Silicified woods composed of opal can easily be distinguished from those composed of chalcedony by three physical properties. In many instances, the less-common opalized wood is not recognized because most people assume that it is chalcedony and testing is not done. Opal has a lower specific gravity, a lower hardness, and a lower refractive index. Any one of these can be used to separate opal from chalcedony.

Specific Gravity2.04 to 2.232.59 to 2.61
Mohs Hardness5.5 to 66.5 to 7
Spot Refractive Index1.39 to 1.481.53 to 1.54

Opalized wood can be just as beautiful as petrified wood composed of chalcedony. However, opalized wood has durability differences and is less suitable for some jewelry and lapidary projects. Opalized wood has a lower hardness and is more easily damaged by abrasion. It also has a lower tenacity and is more likely to break upon impact or exposure to stress.

Opalized Wood

Opalized Wood: A nice piece of opalized wood from Oregon. It is colorful, accepts a bright polish, and shows excellent wood grain. This specimen measures about 3 inches across.

Is Opalized Wood More Valuable?

Some people might hear the name "opalized" wood and assume that it is more valuable than other types of petrified wood. That would be definitely be true if the opal is "precious opal" and exhibits a fine play-of-color. Wood petrified by precious opal does exist, and fine specimens can sell for extremely high prices.

However, most opalized wood is common opal, and the seller often does not know that it is opal (rather than chalcedony) because testing was not done. An argument can be made that opalized wood should sell for a lower price because of its potential durability issues.

As with most gems, color, pattern and beauty are typically what determines value. If an especially beautiful specimen of opalized wood is found and made into a pin, pendant or other item of jewelry where durability is less of a concern, it could justifiably be sold for the high price that its beauty deserves. Beautiful specimens of wood petrified by precious opal are known to sell for very high prices. They have both the beauty of precious opal, plus the interesting aspect of being an organic gem.

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