Fire Opal is sometimes found filling cavities in rhyolite. Long after the rhyolite has cooled, silica-rich ground water moves through the rock, sometimes depositing gems like opal, red beryl, topaz, jasper, or agate in the cavities of the rock. This is one of many excellent geological photographs generously shared through a Creative Commons License by Didier Descouens.

Peridotite is a coarse-grained intrusive igneous rock that is composed almost entirely of olivine. It may contain small amounts of amphibole, feldspar, quartz, or pyroxene. The specimen shown above is about two inches (five centimeters) across.

Pumice is a light-colored vesicular igneous rock. It forms through very rapid solidification of a melt. The vesicular texture is a result of gas trapped in the melt at the time of solidification. The specimen shown above is about two inches (five centimeters) across.

Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. The specimen shown above is about two inches (five centimeters) across.

Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock. It forms near the margins of a magma chamber during the final phases of magma chamber crystallization. It often contains rare minerals that are not found in other parts of the magma chamber. The specimen shown above is about two inches (five centimeters) across.

Gabbro is a coarse-grained, dark-colored, intrusive igneous rock that contains feldspar, pyroxene, and sometimes olivine. The specimen shown above is about two inches (five centimeters) across.

Granite is a coarse-grained, light-colored, intrusive igneous rock that contains mainly quartz, feldspar, and mica minerals. The specimen above is about two inches (five centimeters) across.

What is Diabase? In the United States and Canada, the name “diabase” is used for a dark gray to black, fine-grained, intrusive igneous rock that has a composition similar to basalt and gabbro. The difference between basalt, diabase, and gabbro is in their grain size – which was determined by their cooling rates. Basalt: rapid cooling in a lava flow produced…

Diorite and andesite are similar rocks. They have the same mineral composition and occur in the same geographic areas. The differences are in their grain sizes and their rates of cooling. Diorite crystallized slowly within the Earth. That slow cooling produced a coarse grain size. Andesite forms when a similar magma crystallizes quickly at Earth’s surface. That rapid cooling produces…

Diorite is the name used for a group of coarse-grained igneous rocks with a composition between that of granite and basalt. It usually occurs as large intrusions, dikes, and sills within continental crust. These often form above a convergent plate boundary where an oceanic plate subducts beneath a continental plate. Partial melting of the oceanic plate produces a basaltic magma that…