Central Oregon Rockhound Materials
Agate is a translucent cryptocrystalline variety of quartz and is the most common and variable semiprecious gemstone. It often forms in fractures or other cavities in volcanic rocks. Where weathered out of its host rock, agate is found in concentrations or scattered widely as "float." The local forms are usually white to colorless, often with a layered appearance. Those that contain a distinctive swirled grain such as "angel wing" or tube agate are more highly prized. The best contain designs formed by mineral or other substances which have become a part of the agate itself. These are known as plume, moss, sagenite, dendrite, etc. They may contain what appear to be miniature trees (dendrites) or other plants embedded under the surface. In semiprecious gem stones, it is often the "impurities" that make them distinctive and desirable. Unfortunately, it is often necessary to cut or polish the material to see how good it is. Many rockhounds consider this part of the fun.
Thunder eggs are the most distinctive and one of the most sought after stones in Oregon. On March 5, 1965, it became the official state rock. Thunder eggs are spherical (ball-shaped) masses of rock that range in size from less than an inch to several feet in diameter. Most are a little larger than a baseball. The exterior surface is an uninteresting, drab rind, often with a knobby appearance. It is the core which holds the prized material. When sawed open, the interior may be filled with crystals, agate, jasper, a powdery calcite, or there may be just an empty cavity. Many thunder eggs reveal exquisite and colorful designs ranging from five-pointed stars to miniature landscapes.
Jasper is an opaque variety of cryptocrystalline quartz similar to agate that also occurs in a wide variety of colors. Red and yellow varieties are common while green is more scarce. "Morrisonite" jasper thunder eggs may have tan, dark brown, or even purple cores. Rough jasper is generally dull and must be moistened or polished to bring out the color and sheen. "Picture jasper" is most prized for the picture-like designs which may appear on the cut surfaces. A material similar in appearance, in colors of blue, green, and brown, is locally known as vistaite. It is rare, and is a highly silicified form of rhyolite rather than true jasper.
Limb casts are deposits formed in cavities in volcanic ash which were once occupied by pieces of wood. The hot ash caused the wood to completely burn out, leaving cavities or molds in the shape of the wood. At a later time, agate was deposited in the cavity (like a mold) in the same shape as the original limb, hence the name "limb cast." Many specimens exhibit detailed bark, knot, and wood textures on the surface. Limb cast with a green or pink hue are highly prized.
Rockhound sites to find this material include: Paulina (Congleton Hollow/Dendrite Butte).
Obsidian, a volcanic glass, is perhaps one of the easiest rock materials to identify. Brittle and usually black in color, it exhibits an extremely shiny surface when broken. The most desirable obsidian types include gold sheen, silver sheen, fire sheen, rainbow, midnight lace, double flow, and mahogany. Chips or broken edges are as sharp as broken glass, and should be handled with care. The area around Glass Buttes has been designated for recreational obsidian collecting; mining claims may not be filed for obsidian or chalcedony (agate).
Rockhound sites to find this material include: Glass Butte.
Petrified wood can best be identified by the grain pattern of the annual growth rings, especially when viewed from an end, where the circular form of the rings is apparent. Local petrified woods vary from white to black, but browns are the most common color. Occasional specimens are colorfully agatized and take an excellent polish. Some localities even offer highly prized green-colored petrified wood.