Petrified Wood Colors and Mineral Composition
Petrified wood is not really a wood as we know it. Its name that derives from the Greek petro meaning rock, stone reveals that it is actually a wood turned into stone. Although the shape and texture, and sometimes even color of petrified wood create an illusion of real wood it is actually a mineralized fossil of the original terrestrial vegetation, most often parts of trees.
Petrified wood is formed by a natural process known as petrifaction during which the minerals replace the wood. Once the tree has died it was covered by water and then by a thick layer of mud which prevented its decomposition (wood exposed to the air decomposes and disappears) and enabled the minerals to enter the wood. Over time, the minerals in the water and mud replaced the wood and literally transformed it into stone.
When a tree falls it needs to be covered by mud very quickly otherwise it begins to decompose. The required time to transform wood into petrified wood is a matter of debate. Most examples of petrified wood that were found all over the world were formed millions of years ago but according to some experts, the petrification process could complete as early as in one century only.
The minerals that replace the wood during the petrifaction process take on virtually identical shape, size and texture of the original plant material which is why petrified wood appears like real wood. Even more, in some cases are preserved exceptional details such as tree rings and even cell structures of the original organic material. But petrified wood is perhaps even more valued for its rich colors.
The color of petrified wood depends on the chemical composition, more specifically minerals present in water and mud that covered the tree during the petrifaction process. The most common is silicon dioxide (SiO2). Petrified wood that is composed of this mineral is often referred to as silicified wood and is characterized by white color. However, petrified wood made of pure silicon dioxide is very rare as other minerals are typically present in water and mud as well. For that reason silicified wood is most often brown and reddish brown. White color can be also created by calcite but in this case petrified wood eventually darkens if exposed to sunlight. Very common is also quartz, the second most common mineral in the Earth’s crust. Quartz is colorless which is why it creates an illusion of “natural” wood color making petrified wood appear even more real.
Some types of petrified wood contain a variety of colors which makes them highly valued as decoration products. Colorful petrified woods are predominantly composed of silicon dioxide but they contain one or multiple minerals other than silicon dioxide such as iron, copper, manganese, cobalt, iron oxides, manganese dioxide, pyrite, goethite and calcite. The intensity and number of colors depends on the amount and type of the mentioned minerals that were present in water and mud that covered the tree during the petrifaction process.
Brown and reddish brown color indicate presence of iron which turns reddish brown when exposed to the air. The more the iron the more intense the red color. Green and blue color can be a result of several minerals including copper, cobalt and manganese, while manganese dioxide can produce blue, pink or purple color. Again, the higher the amount of any of the mentioned minerals the more intense the color. Brown, yellow and orange can be produced by goethite (an iron oxyhydroxide), while black color indicates presence of pyrite.
Both the color and mineral composition of petrified wood depend on chemical composition of the water and the mud at the time when the tree went through the petrifaction process. Regardless of color and mineral composition, petrified wood is very fragile yet hard as a rock. It has a Mohs hardiness of 7 which equals the hardiness of quartz, the main element of glass. Particularly valued are petrified woods that are composed either of rare minerals or multiple minerals giving it a an array of colors. These are very beautiful and are also very popular for decoration purposes because they are easy to cut, polish and shape, and create unique pieces of jewelry as well as interior and exterior decoration products and even pieces of furniture.