Petrified wood is considered to be a gemstone, and while it is technically a stone, it did not originate as one. Petrified wood begins as a tree or a plant which is similar to a tree in structure which then undergoes a process known as permineralization. As this process unfolds, the tree is fossilized.
The organic material of the tree is gradually replaced with minerals, which still take on the form and structure of the original organic matter. This is why a piece of petrified wood still resembles the tree which originally existed, much as the fossil of a dinosaur bone or a sea creature will still resemble that ancient creature.
The difference however is that fossilized animals typically are flat impressions left by organic material (like a mold of the original), whereas petrified wood is fully three-dimensional.
There are two ways you can delineate between different types of petrified wood. You can either classify the wood based on the species of tree which was petrified, or based on the minerals which have taken over.
Any type of wood can become petrified, be it redwood, oak, maple, or anything else. Prehistoric trees have been petrified and preserved as well. Patterns in a piece of petrified wood can assist you in determining where the wood came from, as can knowing where the wood is from. For example, if you know that a piece of petrified wood came from the northern California coastline and its structure resembles redwood, that is probably the type of petrified wood it is.
Cell structure offers another means of identification. You may need to magnify as much as 800x in order to examine the cell structures of petrified wood. These structural patterns can shed more light on the type of wood you are looking at. You can also search for resin ducts (found for example in pine) and other unique identifying features.
What types of minerals are involved in the formation of petrified wood?
Here there can be some variation, but typically you are looking at a silicate like quartz. The different colors which emerge during the petrification process can also tell you something about the elemental composition of the wood.
Green and blue traces may indicate the presence of chromium or copper, whereas iron oxides may tint wood yellow, brown, or red.
Manganese and manganese oxides can contribute pink, orange, yellow or black, and carbon may also leave black markers. Newly petrified wood is often white before it is exposed to the elements since quartz silicate is white.
One special “type” of petrified wood (a bit of a misnomer, as you will see) which is uniquely prized is jet. Jet is a mineraloid and not a true mineral, and is actually a type of super-compact coal derived from decaying wood. In this sense it is not a true petrified wood either, since it is actually formed from a decomposition process of wood and not a replacement process, retaining its organic nature millions of years later.
Jet cuts very easily and is quite soft, which makes it easy to carve. Many pieces of jet which are used in jewelry have been very finely and intricately carved as a result, and can produce quite an ornate effect. Jet was a favorite material for jewelry in ancient Rome, and while it is no longer as popular as it once was, it is still treasured for its age, softness, and beauty. As the name implies, it is jet black in color. Actually, it’s the stone itself that gave us the phrase “jet black.”
There is a great deal of confusion surrounding the classification of jet online, but now you hopefully understand why jet is considered to be related to petrified wood, but is not actually a form of it. Petrified wood is classified as a rock, since it contains an amalgam of minerals. While jet has similar origins, it is not actually a rock or a mineral at all, but a mineraloid gemstone like amber, opals or pearls.
And next time you do see a piece of actual petrified wood, you will hopefully be able to identify something of its elemental properties from its colors, and maybe even what type of wood it is according to the preserved structure of the grain.