Extrusive igneous rocks form after lava cools above the surface. Extrusive igneous rocks cool much more rapidly than intrusive rocks. There is little time for crystals to form, so extrusive igneous rocks have tiny crystals (Figure below). Cooled lava forms basalt with no visible crystals.

If magma cools quickly, for example when basalt lava erupts from a volcano, then many crystals form very quickly, and the resulting rock is fine-grained, with crystals usually less than 1mm in size. If magma is trapped underground in an igneous intrusion, it cools slowly because it is insulated by the surrounding rock.

Test whether the rock has a hollow interior. Pick up the rock and assess its weight. If the rock feels lighter than that surrounding rocks, it may be a geode. Geodes have a hollow space inside, which is what allows the crystals to form. You can also shake the rock next to your ear to test whether it is hollow.

Cool rocks can be found in landscaping gravel, construction sites, and road-cuts in your neighborhood. In nature, look for cool rocks near hills, mountains, forests, outcrops, lakes, and streams. Keep reading for more information on where to look in these areas.

Any mineral or gemstone that forms as a result of geologic processes can be found in stream beds. Quartz is the most common crystal on earth and is often found in streams. It’s not uncommon to find amethyst, jasper, topaz and beryl in stream beds.

What are Precious and Semiprecious Stones? All of these stones belong to the “semiprecious stones” category. Included in the photo are gold tiger’s-eye, gray Botswana agate, carnelian agate, fancy jasper, red jasper, green tree agate, sodalite, plasma jasper, blue chalcedony, eye agate, blue lace agate and others.

Garnet Value, Price, and Jewelry Information

• Hiddenite, North Carolina.
• Murfreesboro, Arkansas.
• Spruce Pine, North Carolina.
• Franklin, North Carolina.
• Philipsburg, Montana.
• Amelia, Virginia.
• Virgin Valley, Nevada.
• Denio, Nevada.

Cut and polished, a ”star garnet” is an attractive burgundy-colored jewel worth $10 to $125 a carat. There are only two places in the world where star garnets have been unearthed: northern India and the Emerald Creek area of the Idaho Panhandle National Forest.

Places to Dig for Gems Idaho is known as the “Gem State” because it contains 72 different kinds of precious and semi-precious stones. Idaho gems can be found throughout the state, particularly in mining sites and stream beds.

Collecting is not specifically prohibited or restricted. Your collecting activities result in no more than minor disturbance of the public lands. Equipment is limited to hand tools and metal detectors. You may not use explosives or motorized/mechanical devices such as earthmoving equipment.

The most important gems produced to date have been garnet and opal. Notable amounts of jade, topaz, zircon, and tourmaline have also been found in Idaho. Agate, jasper, and petrified wood in many colors and patterns have been found and produced from small deposits in many parts of the state.

According to the Gemstone Guide on the Idaho Department of Lands website, quartz-lined geodes can be found in the upper valley of Lost River in Custer County, near the DeLamar Silver Mine in Owyhee County, and northwest of the town of Weiser, Idaho in Washington County.

The small town of Spencer in eastern Idaho is often referred to as “The Opal Capital of America”. After all, it’s here you’ll find the largest opal mine in the state—Spencer Opal Mine—where you can dig for your own opals.

In Idaho, Opal is the second largest contributor to the total value of gem material produced. The varieties produced include precious (white and pink), yellow, blue, pink, and common. The Spencer Opal Mine, the largest privately owned gem stone producer in the State, is the major producer of Opal.