Some of the best specimens of petrified wood have been preserved by silicification. Two forms of silicification are common. The most abundant is wood that has been replaced and infilled by chalcedony (sometimes called “agatized wood”). The other form is wood that has been infilled and replaced by opal (usually called “opalized wood”). Both of these varieties can be called “silicified wood” if you are not certain of their identity.

These materials can have a similar appearance that requires testing to positively identify. However, if you have experience in geology or gemology, the tests below are very helpful in separating them. Opalized wood has a lower hardness, a lower refractive index, and a lower specific gravity than chalcedony, as shown in the table below.

	Opal	Chalcedony
Specific Gravity	2.04 to 2.23	2.59 to 2.61
Mohs Hardness	5.5 to 6	6.5 to 7
Spot Refractive Index	1.39 to 1.48	1.53 to 1.54

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.