San Diego Natural History Museum Mineralogy Department

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FAQs

What kinds of minerals occur in our region?

San Diego County is about 900 square miles of very complex geology, from the high desert and mountains to the coast with lots of faulting all along the way. So a lot depends on where you are looking. Here is a list of many of the minerals known to occur in the county:

Acmite, Albite, Allanite, Amblygonite, Andalusite, Apatite, Arsenopyrite, Azurite, Bavenite, Bertrandite, Beryl, Biotite, Bismite, Bismuth, Bismuthinite, Bornite, Calcite (optical), Cassiterite, Celestite, Cerussite, Chalcocite, Chalcopyrite, Chrysotile, Clintonite, Cookeite, Corundum, Epidote, Erythite, Ferrimolybdite, Ferrisicklerite, Ferroaxinite, Fersmite, Fluorapatite, Francolite, Gahnite, Galena, Garnet, Glauconite, Gold, Graphite, Gypsum, Helvite, Heterosite, Heulandite, Hydromagnesite, Laumontite, Lawsonite, Leadhillite, Lepidolite, Limonite, Lithiophylite, Magnetite, Malachite, Manganite, Marcasite, Microcline, Molybdenite, Morenosite, Morinite, Muscovite, Nickel, Orthoclase, Pentlandite, Petalite, Plagioclase feldspar, Pollucite, Purpurite, Pyrite, Pyrophyllite, Pyrrhotite, Quartz, Rhodonite, Rutile, Rynersonite, Samarskite, Scheelite, Sicklerite, Silver, Sphalerite, Spinel, Spodumene, Stellerite, Stokesite, Tellurium, Tenorite, Thorogummite, Todorokite, Topaz, Tourmaline, Tremolite, Tridymite, Triphylite, Uranmicrolite, Uranophane, Violarite, Wollastonite, Zircon


Did you say gold?

Of course. This is California, after all. Yes, there have been some lode and placer deposits of gold in the county, known from as far back as 1828 (see Pemberton's book for more information). You can visit the Eagle Gold Mine in Julian on a tour and learn how to pan for gold.


What's a pegmatite?

Picture granite as a mass of hot molten material (magma) below the surface of the earth, cooling slowly into solid rock. If part of the magma contains a lot of water, it will hold more dissolved minerals, and it will cool even more slowly than the surrounding granite. The effect of this very slow cooling of mineral-rich magma is the formation of pockets of large crystals (such as mica, quartz, tourmaline, and lepidolite) within the granitic rock, commonly found with all the crystals pointing inward. These bodies of rock within a granite mass with large crystals are called pegmatites. Most of the spectacular crystal-producing areas in the world, including Minas Gerais (Brazil), Sri Lanka and San Diego County, are pegmatite areas. Crystals up to 10 meters (33 feet) in length have been found in pegmatites.

In San Diego County, pegmatite deposits have yielded famous and spectacular gemstone crystals, including many varieties of tourmaline, beryl, topaz, kunzite, and spodumene. Trace elements, such as boron and beryllium, affect the formation of different types and colors of crystals. Several areas in the county are especially famous for production of world-class tourmalines (though most of these are now closed).


Just how many kinds of tourmaline are there?

There are several main types: schorl, dravite and elbaite. The colors can range from pale pink and blue to black, and include the pink and green banded variety you may have heard called "watermelon." A single specimen may show an amazing range of colors. Schorl is black or very dark and contains iron; dravite is brown and contains magnesium. Elbaite contains lithium and sodium, and comes in a range of colors, including green (verdelite), yellow, red to pink (rubellite), and blue (indicolite). Rarely, tourmaline may be white or even colorless (achroite). San Diego County has produced gem-quality tourmalines for many years.


Where and how can I collect minerals?

If you're not familiar with the area or with field collecting, start with a local museum. The San Diego Natural History Museum, the San Bernardino County Museum, and the Fallbrook Gem and Mineral Society Museum offer programs in geology for all age levels.


Can I visit a mine?

There are very few mines available for open collecting in San Diego County. Many mines are extremely dangerous, especially if they have been closed for a while. Please stay away from them. If you are interested in learning about the history of mining in the northern part of San Diego County, or in a guided trip to a mine, please contact the Fallbrook Gem and Mineral Society Museum


Where would I learn how to do lapidary work?

Contact the San Diego Mineral and Gem Society. They have several sections and lots of opportunities for you to learn how to tumble, polish and shape stones safely.


What's the difference between a rock and a mineral?

A mineral is a naturally occurring, inorganic, crystalline solid having characteristics, physcial properties, and a narrowly defined chemical composition. A rock is a naturally formed consolidated material composed of grains of one or more minerals.


I think I've found a meteorite. How can I find out for sure?

Sometimes it's easy; sometimes it takes some detective work. You'll need to talk to an expert to be sure. Meteorites can be divided into three groups based on their composition: stony, or composed primarily of minerals; stony-iron, or composed roughly equally of minerals and native metallic iron with varying amounts of nickel; and iron, composed principally of metal (iron and nickel). Fresh meteorites often show a dark surface, or fusion crust, from their trip through the atmosphere. Stony-iron and iron meteorites are heavy for their size. Stony meteorites may not look particularly different from other rocks. Often meteorites simply look very different from anything else in the vicinity. There are lots of substances that mimic meteorites, though, including pyrite nodules and industrial slag. If you think you have found a meteorite and want to find out more about it, one good place to start is the University of New Mexico Institute of Meteoritics.


Do you have classes in gemology?

We're in the right part of the world for them! The Gemological Institute of America headquarters is in San Diego County, near Carlsbad. GIA offers an extensive range of professional courses in gemology.


What are geodes, and where can I find one?

Geodes form in several different ways but are most common in sedimentary rocks. Geodes are essentially open spaces like rock cavities or vugs partly filled with crystals or concentric bands. The exterior of the most common geodes is generally limestone or a related rock, while the interior contains quartz crystals and/or chalcedony (fine-grained quartz) deposits or any crystal that precipitates from ground water such as calcite or barite. 

Geodes can form in any cavity that is buried. These can be gas bubbles in igneous rocks, vesicles in lava after a volcanic eruption, or even buried bits of organic material from dead sea creatures. Over time, the external wall of the cavity hardens,while dissolved minerals are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater solutions allows well-shaped crystals to form inside the hollow chamber. Eventually, the geode makes its way back to the surface through normal geologic processes such as erosion.

Geodes can vary considerably in size but commonly range up to the size of a melon or soccer ball. Some are as large as a meter across. Since they commonly form in sedimentary muds, they are usually found near the surface. I would search on-line to see if there are any such geode-bearing rocks near you. Good luck.