Garnet crystals are a favorite with mineral collectors and jewelry makers, both. The gem crystals can be faceted into beautiful stones while many of the natural specimens look so glossy and smooth that many casual observers will imagine the raw crystals to be the result of polishing. Of all the garnet species found, the variety, Spessartine, is found as well formed, gem-like crystals, the result of rhyolite uplifts across the western United States.
Rhyolite is an igneous rock form created by silica rich lava flows, with a rich mixture of quartz, feldspar, flakes of biotite mica and hornblende. In many of the rhyolite uplifts across the western united states, topaz and spessartine are found hand in hand, in the empty gas voids spread through the rock. In addition, rhyolite serves as the host to obsidian, pumice, thunder-eggs, geodes and many other things rockhounds love, making every rhyolite deposit worthy of investigation. Several volcanic tubes can be identified across Arizona and New Mexico that resulted in deposits of spessartine garnet and other minerals. The garnets found at these locations tend to be 2 centimeters or smaller in size, making faceting rough a hard thing to come by, however, the natural crystals found in these deposits are popular in wire wrapped jewelry designs.
The formation of the crystals in these rhyolite deposits are interesting to dissect, as the necessary elements are found in addition to the elements in the minerals forming the stone. The basic elements found in the rhyolite rock type include silica (Si), aluminum (Al), fluorite (F) and manganese (Mn), which are all the necessary elements required to form crystals of topaz Al2SiO4F2 and spessartine Mn++3Al2(SiO4)3. The rhyolite pockets, or “lithophyses”, are quite a sight to be seen, formed out of thin rounded layers of rhyolite, layered like petals of a delicate stone flower, sprinkled with sparkling druzy quartz, finally finishing with a chance of hosting a few crystals of spessartine garnet or topaz.
In New Mexico we have a rhyolite plug occurring along a dark black sloping basalt mesa in the western part of the state. The ease of access to the mineral rich area makes Grants Ridge a popular collecting location, just outside of the city of Grants, New Mexico. In the Eastern section of Grants Ridge the rhyolite is studded with cavities up to 10 cm wide, lined with small crystals of clear topaz and gem red spessartine garnets. This deposit spreads out over a large area, comprised of three layers of volcanic flow, two of which contain the crystals of garnet and topaz. Splitting the rhyolite open is done with ease, revealing the sparkling vugs of crystals distributed through the rocks. The spessartine garnets found here are bright red and full of internal fire. Finding anything large enough to facet would be quite challenging, but the voids are easy to break open The garnets and topaz are already sparkling when a cavity is exposed and little cleaning is required beyond a general washing with water.
Further south in New Mexico, a string of rhyolite plugs spring up in the Black Range, carrying with them the elements to form spessartine, topaz, cassiterite and bixbyite, The sandy gray rhyolite forms thin shells which reveal cavities lined with crystals. The deposit in the Black Range is very similar to the classic topaz rockhounding location in the Thomas Range of Utah. There have even been finds of small crystals of red beryl, thought to only form in the Wah Wah Mountains of Utah.
Across the state line in the mountains southeast of Hayden, Arizona is a rhyolite deposit that produces larger crystals with a possibility of producing small facet rough for gemstones. The spessartine crystals of Ash Creek, a mile and a half outside of the town of Hayden, Arizona, can be found as large as 3.7 cm, with a deep red color shifting to a beautiful orange color, caused by minute inclusions of quartz crystals forming imbedded in the surface of the crystal faces. Accessing the location can be reserved for only the most dedicated rockhounds, as one visitor described it as “The Table Lands”. The satellite map of the location shows huge mesas stretching out over a rugged rocky mountain, outlined with creeks filled with banks of shrubbery and trees.
The final location for spessartine garnet in Arizona is in the Aquarius Mountains, just a few miles east of route 93 in Mohave County. The purplish gray rhyolite deposits are not as full of lithophyses as the New Mexico locations, with much thicker layers of rhyolite in between cavities. The spessartine garnets from here are some of the finest, with such smooth terminations that they look like polished gemstones glued onto a rock. The spessartine crystals can grow to over an inch at this location, sometimes classically rounded, but often found as flattened spheres, clinging to the surface of the rhyolite.
Garnets, like spessartine, are identified not by their color, rather, the chemical composition. Many locations where spessartine garnets were thought to form turned out to have more iron than manganese, which would make them almandine garnets. The spessartine crystals from Grants Ridge, the Black Ridge, and Ash Creek are without a doubt members of the spessartine division of garnet species, however, the garnets from the Aquarius Mountains can vary over the deposit, with some of the garnets testing as iron rich and others testing as manganese rich, even though they are visually similar!
With all the needed characteristics to be a great gemstone and mineral specimen, these volcanic deposits of rhyolite serve as a great rockhounding opportunity. The wide open spaces of Arizona and New Mexico provide the perfect setting for these deposits.
First photo Lion Springs Garnet, courtesy of Brandy Zzyzx