See locality: Unnamed Corundum Occurrences, San Jacinto Mountains, Riverside Co., California, USA

Reprinted from the Lithosphere bulletin, May/June 2000.
© 2000 Fallbrook Gem and Mineral Society, Inc., Fallbrook, California. Used with author's permission.

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The Rediscovery
of Corundum Crystals
from the San Jacinto Mountains,
Riverside County, California

By Kenneth L. Gochenour

I first saw specimens of corundum crystals from the San Jacinto Mountains in 1980 at the booth of Mark Rogers during the Pasadena Mineral Show.

Their symmetry and rare morphology immediately intrigued me. The crystals were doubly terminated, spindle shaped and nearly 12 inches in length. Some specimens exhibited multiple crystals in a schist matrix. I soon found that the crystals were collected in the late 1940's by an "Oriental couple." Unfortunately, the exact Locality was unknown.

My brother Dana and I were able to get some idea as to the locality so we set out to try to find it. We visited the presumed locality several times after the show but we only found a small outcrop of the corundum-bearing schist that contained crystals up to 4 inches in length. These crystals were nothing like the crystals that we had seen at the show. We often dreamed of finding and resurrecting this classic locality. We did find some fine grossular garnets nearby and satisfied ourselves with them rather than beat the very dense brush while trying to locate the corundum.

October 1999.

Just before the Pasadena show we again decided to give the corundum locality another try. We began our search at home in the library. Over the years we had been building up bits and pieces of information from quite a few different sources. Bob Reynolds, then of the San Bernardino County Museum, our cousin Bill Rawley, and Mark Rogers all had given us useful bits of information that we hoped would lead us to the right spot. We had also discovered an article in the Mineralogist from the 1950's containing a detailed account of the deposit but it had only vague information as to its location. Since we were fairly sure we were in the general area of the search we used the combined information along with some good geologic maps and a structural geology textbook to try to narrow down the search area.

In the field we began by trying to find the spot that we had found some 11 years previously. The area had of course changed quite a bit. A fire had left the once blush-covered slopes barren. After a systematic search we finally found our original spot. This spot soon showed itself to be not the right spot, at least not the long-lost spot. It just did not correlate with our information. Sometimes there is only one option left to the prospector: try someplace else. So we did.

We set off for the next mountain and soon found some small corundum crystals in schist matrix scattered in the float field below the peak. The crystals were small and badly weathered. This was a good sign that corundum was found in more than one place.

According to our information, there was a limestone outcrop above the corundum locality. The outcrop was said to have some grossular garnet and wollastonite crystals that could be collected. We followed the ridge up to a white rock outcrop that proved to be the garnet locale. We spent the rest of that day exploring for garnets and were able to retrieve some excellent crystals frozen in quartz.

On our next trip we again made our way to the garnet tactite. While Dana looked for garnets, I set off to do some exploring. After several fruitless hours of searching, I was able to locate some of the corundum in schist. Again it was too small and certainly on the wrong part of the hill to be the spot I was looking for. We retreated down to the truck with garnets but nothing more than that and dreams of giant corundum crystals.

It took two more trips before we finally located the original corundum outcrop. The outcrop did not outcrop; instead, the dark biotite schist barely stuck up out of the ground. Here were the remains of an old working and schist with broken crystals that were once as much as 12 inches long and an inch in diameter. The schist had some crystals that were 7 inches long still in place and we spent the day trying to remove two pack loads to take down the hill for preparation and repair. On subsequent trips we found two more spots that had corundum crystals of collectable size and quality.

At locality #1 we found some cleavages of same very large corundum crystals loose in the topsoil. Some of these monster crystals had once been 2 feet long or even longer. The largest crystal was more than 3 inches in diameter. All this material was loose in topsoil and the original schist was not to be seen. We removed over 150 pounds of crystals from this "potato patch."

Basic Geology.

The deposit is on the contact of a Mesozoic quartz diorite and later Paleozoic metasediments. In geologic terms, this is called nonconformity. The later Paleozoic sediments were metamorphosed at the contact with the granodiorite/quartz diorite. Small lenses and sills of schist cross boundaries on the contact. It is in some of these schist bodies that the corundum crystals are found. The garnet tactites are found in this same environment. Because the corundum-bearing schist has a much larger grain size and larger mica crystals, it is less resistant to weathering, thus giving the schist a lower profile. In some cases the schist is rotten and has been reduced to a layer of iron-stained micaceous clays. Veins of quartz and the garnet tactites are found in close proximity to the corundum-bearing schists.

Some small scale prospecting has been done in the past for tungsten ores as evidenced by numerous bulldozer cuts exposing tactite bodies in the surrounding area.

The corundum-bearing schist is a biotite schist with inclusions of quartz and feldspar found nearby the largest corundum crystals. Here the schist is quite coarse and the biotite crystals are enlarged. The crystals of corundum are oriented randomly and found in sprays and bunches in the schist. Non corundum-bearing schists in the area are quite resistant to any weathering and are much more sill-like in their form. The largest corundum-bearing schist lens that has been found at this time is only 20 feet long and 10 feet in diameter with a depth of about 3 feet.

Recovery of the crystals from the schist matrix is difficult because of the toughness of the interlocking mica crystals and also because the crystals have been stressed in their cooling. The crystals cleave into sections and must be restored as each piece is removed from the matrix. The crystals are included with biotite so severely that in some crystals there is partial replacement. This makes recovery very slow and tedious. Great care must be exercised during the preparation process. We hope to find better and more well preserved crystals in the future as the deposit is likely to contain more corundum-bearing schists.

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