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inside of glassy rock
inside of glassy rock  

outside of glassy rock
outside of glassy rock  
I found this rock during a brief stop in Red Rock Canyon, CA, area. I pulled a greenish yellow rock from the side of a dry gulch. The rock broke, and this is the piece I was able to keep. The outside is, as said, yellowish green, coarse, and pitted. Inside is clear glassy to opaque white. The tube running diagonally through the center is at least hollow in parts.

Hi Christopher,
It looks like that you found a beautiful geode with some interesting mineral growth inside of it. These minerals in geodes precipitate from water (or rarely from gas). As water migrates through a cavity or opening in the rock the dissolved minerals slowly precipitate. A very common mineral found in geodes is quartz (SiO2) in its numerous varieties (chalcedony, jasper, agate, chert, even opal) and during the precipitation process can create some really bizarre but pretty forms. This type of quartz is known to geologists as microcrystalline quartz, because it does not show the beautiful rock crystal form, but is made of numerous, super tiny quartz crystals, to small to see with the naked eye or even a powerful microscope. A scanning electron microscope or other means would be needed to see these submicroscopic minute crystals making up forms of microcrystalline quartz.
While it is difficult to give you an exact mineral analysis from pictures alone, I can give you some educated guess of what might be in your geode:
The hollow, tube like structure is probably a chalcedony (microcrystalline quartz) tube, which are interpreted as an indication of flowing watery solutions, perhaps coming out of the pores of the rock wall.
The formation of the bubbly looking crystals, called botryoidal crystal growth, probably formed in non flowing conditions where the mineral precipitates slowly, layer upon layer, from the geode wall inward. It could also be opal, which can be wide. Opal is found in the Red Canyon Area, CA. Opal is basically quartz with some water embedded into its chemical structure (SiO2 nH2O). This added water prevents actual crystal formation, even microscopic crystal and opal is known to be amorphous (without crystal structure). It is hard to tell from the picture if the botryoidal growth in your geode is opal (if it is glassy, jelly like looking it probably is). To know 100% for sure will take some more sophisticated instrumental analysis.
The chalky white crystal at the edge of the geode might be a zeolite, a mineral group that contains silica and water. Zeolites have been also reported from the Red Rocks Canyon area, CA. It might also be calcite (CaCO3) or calcium carbonate. To test for calcite, put a drop of strong vinegar on the crystal. Calcite will start to effervesce or bubble when exposed to acid. There is also the possibility that this is gypsum (CaSO4 * 2H2O). Gypsum would be really soft and could be scratched with your fingernail. Unfortunately without a physical investigation of your geode it is impossible to tell for sure.
Hope this helps. If you really want to know for sure, our university does a FREE mineral analysis for the public as an educational outreach. A student assigned in my mineralogy class would analyse your specimen and write you a very detailed report. Of course you would get your sample back. This process would take about one semester and I would need your sample by end of August, beginning of September to be assigned to the Fall 2016 course. Details about this service can be found here:  


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Uwe Richard Kackstaetter, Ph.D. (Dr.K)


I can answer questions concerning minerals, mineralogy, gems, metals, groundwater, national and international geoscience field trips and anything that has to do with geology. As a public service and part as training for new geoscientists, our university department provides detailed FREE mineral identification for individuals with available non-destructive and destructive analytical procedures and a several page report. Please contact me for details or go to for details..


I am a professor of applied geology and mineralogy with many hours of field experience. Furthermore, I enjoy recreational gold prospecting and mineral collecting. As a professor I am engaged in research concerning minerals, their identification and their occurrence. I am currently developing inexpensive, accurate mineral identification procedures to be used by experts and lay people alike.

Member of the GPAA (Gold Prospectors Association of America) as well as the Association of Environmental Geochemists. Member of the GSA (Geologic Society of America) Member of the AIPG (American Institute of Professional Geologists)

Here is a small sampling: Mineral-rock handbook: Rapid-easy mineral-rock determination : written for anyone interested in minerals and rocks - Proctor, Peterson, and Kackstaetter;Macmillan Pub. Co. (New York and Toronto and New York) Physical Geology Laboratory e-Manual with 20 Lab Exercises [now available for FREE]: Colorado Front Range Self-Guided Geology Field Trips [FREE download] Kackstaetter, U.R. (2014): A Rapid, Inexpensive and Portable Field and Laboratory Method to Accurately Determine the Specific Gravity of Rocks and Minerals. The Professional Geologist, Vol. 51, No.2, AIPG. Kackstaetter, U.R. (2014): SEDMIN - Microsoft Excel (TM) spreadsheet for calculating fine-grained sedimentary rock mineralogy from bulk geochemical analysis. Cent. Eur. J. Geosci. DOI: 10.2478/s13533-012-0170-3

Ph.D. in Applied Geology and Mineralogy. I am actively teaching courses in mineralogy, igneous & metamorphic petrology, applied volcanology and a variety of national and international field courses with mineral collecting opportunities. Background in precious metal exploration and groundwater (hydrogeology).

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