Silica – Agates & Opals & Quartz- Oh My!
By Helen Serras-Herman
This elaborate title Silica – Agates & Opals & Quartz- Oh My! is the theme for this year’s 68th Annual Tucson Gem & Mineral Show® sponsored by the Tucson Gem & Mineral Society. The show is held at the Tucson Convention Center, February 9-12, 2023. Lapidaries and collectors can find many agates, opals, and quartz specimens at that show, as well as spread throughout all the gem and mineral shows in the Tucson 2023 Showcase.
Huge drusy quartz specimen from Brazil on exhibit at Tucson, Photo Helen Serras-Herman
The title hints at the chemical composition of silica (SIO2) binds quartz, agates, and opals together. Quartz is one of the largest mineral groups, with over 100 different varieties. Quartzes are among the most popular gem materials utilized by lapidaries and gem carvers worldwide. Their hardness, wide range of textures and color varieties, crystal sizes, durability, availability, and often affordability, make the quartz family of gemstones irresistible and most attractive.
Quartz is divided into two main groups: crystalline and crypto-crystalline (crystals too small to be seen under the microscope). Crystalline varieties include rock crystal, amethyst, ametrine, and smoky quartz, where crystals are frequently well-defined with terminations, often occurring in clusters, and can range in size from thumb-size to mega-crystals. The crypto-crystalline varieties include the chalcedony group, agates, jaspers, flint and chert, and chalcedonic replacements, such as tiger eye or petrified wood.
Large rose quartz spheres on exhibit at Tucson, Photo Helen Serras-Herman
The hardness for all quartzes is 6.5-7.0 Mohs. Quartz is usually considered the benchmark that separates hard stones from soft ones. The crypto-crystalline varieties are somewhat lower in hardness than the crystalline varieties, but they gain in strength as the hidden, fine-grained structure of microscopic interlocking crystals makes these varieties more compact and tough. The hardness is high enough to allow for intricate carvings and engravings, high polish, and good wearability for jewelry pieces. The crypto-crystalline varieties commonly occur in veins, nodules, massive, and drusy or botryoidal crust formations.
Chalcedony varieties show fewer patterns than their “cousins” the agates and jaspers. They may show hue and tone variations within the primary color, but they are best known for their uniformity. Prominent chalcedonies include chrysoprase, cornelian, blue chalcedony, and gem silica.
Agate and white drusy quartz, gem sculpture “The Visionary” by Helen Serras-Herman, Photo Michael J. Colella
Agate is a variety of crypto-crystalline quartz, usually exhibiting banded color formations, moss or dendritic inclusions, zig-zag lace bands, and fire agate. Agates range in value from very inexpensive, which are often dyed to produce vivid colors, to very rare collector’s specimens.
We slabbed this complete agate nodule. It is now part of the Carnegie Museum of Natural History collection for education. Photo Helen Serras-Herman
Opals are another form of silica, but they differ from quartz because opals are non-crystalline silica with high water content, usually 3-20%. Opals have a different structure consisting of closely packed round spheres of silica of fairly regular size. Light reflects and refracts and causes interference and diffraction as it travels through the spheres. The different size spheres produce different colors.
This extraordinary 3.25-carat black opal, from Lightning Ridge, Australia, cut by Andrew Herman, displays chatoyancy in addition to play-of-color. Photo Helen Serras-Herman
Black and white opals are famous because they display the optical phenomenon, play-of-color. Boulder opals display the same seams of opal resting on or penetrating the ironstone matrix, and many Mexican fire opals produce the same.
There are many opal varieties, qualities, and prices to match our aesthetics and budget. I love working with and showcasing unusual opals. I also find irresistible carving exotic common opals, which, for the most part, do not show play-of-color. Their striking body colors — neon blue, pink, purple, yellow, green, and bands and stripes — produce breathtaking scenic patterns. Varieties of exotic common opals include blue and pink Peruvian opal, ice-cream opal, Morado opal, bertrandite (Tiffany Stone), candy-stripe and Utah lace opal, dendritic opals, yellow, and many green opals. Customers find these exotic pieces alluring and fascinating.
Blue Peruvian opal is a beloved exotic common opal featured in these earrings. Photo Helen Serras-Herman
There is a certain amount of heat sensitivity for all of the quartz materials during carving, sanding, and final polishing. That danger is somewhat reduced in the chalcedony varieties. There is little to no cleavage problem, however, directional and varying hardness within the material may be a problem in the polishing stage and may produce a rippled finish if not enough care has been taken during the sanding stages. Natural fractures are common, and it is best to work around them or saw the material right down the fracture line.
Lapidaries and carvers often achieve stunning design combinations by taking advantage of color variations, dendritic inclusions, outside rind and matrix, and contrasting surface textures — smooth and drusy.
Ametrine quartz with its two bold colors is a favorite among carvers.
This one is carved by David Christensen, Photo David Christensen
I am often asked whether there is a difference in carving opal compared to other colored stones. There are no fundamental differences in carving any gemstone. The lapidary steps are generally the same: grinding, sanding, more sanding, and final polishing. That being said, each material has a critical lapidary grit stage depending on the material’s hardness and uniformity.
What makes cabbing and carving opals differ from other gemstones is that the precious opal bands are often hidden deep into the stone and must gradually be revealed. The color bands may also be very thin and vanish in a flash with aggressive grinding, therefore, a slower grinding approach of “grind and look, and look some more” is highly recommended. My husband, Andy, loved the “treasure hunt” segment of cutting opals — looking for and exposing the color bands.
68th Annual Tucson Gem & Mineral Show®
I hope you will not miss the 68th Annual Tucson Gem & Mineral Show®. Besides the over 250 fabulous dealers, stop by and enjoy all the amazing collections brought in by museums and private collectors as guest exhibits honoring the show’s theme. My guest exhibit case will showcase my “Gem Portraits in Quartz”, and I will be present at my Gem Art Center/Helen Serras-Herman Booth #1601 in the Ex Hall.
Huge amethyst geodes are always Tucson’s showstoppers. Photo Helen Serras-Herman
COVER PHOTO: This portrait of George Washington in a 10.5 lbs amethyst will be on exhibit at the
68th Annual Tucson Gem & Mineral Show®. Photo Michael J. Colella
Helen Serras-Herman, a 2003 National Lapidary Hall of Fame inductee, is an acclaimed artist with 40 years of experience in unique gem sculpture and jewelry art. See her work at www.gemartcenter.com and her business Facebook page at Gem Art Center/Helen Serras-Herman