Welcome to Geocaching HQ
At the plaza outside of Geocaching HQ, you’ll find several light-colored crystalline stones set into the brickwork around the Late for the Interurban statue. These stones showcase a variety of minerals, making them a perfect spot to explore the colors, shapes, and textures of natural crystals. This EarthCache invites you to observe the individual mineral grains and learn how each contributes to the speckled patterns you see.
Minerals to Observe
The stones here contain several distinct minerals, each with characteristic appearance.
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Quartz – Translucent to milky grains with a glassy shine, irregular in shape, usually colorless to light gray. Quartz is very hard and resistant to scratching, making it durable in both natural and polished stones. It does not show cleavage and often forms irregular patches or rounded grains within the rock. Quartz contributes to the sparkling, glassy appearance.
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Feldspar – Blocky crystals in white, gray, or pale pink, forming most of the light-colored portion of the stone. Feldspar can show small lines or striations on crystal faces and sometimes slightly weathered surfaces. It makes up the bulk of the rock and gives it the solid, blocky look. There are usually two types present: potassium feldspar (pinkish) and plagioclase feldspar (whitish), which can be distinguished by subtle color and shape differences.
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Biotite – Small, flat, shiny black or dark brown flakes scattered throughout the stone. Biotite is reflective and can shimmer in sunlight. The flakes are thin and layered. Biotite adds dark contrast to the lighter minerals, creating the speckled “salt-and-pepper” pattern. It may also oxidize over time, producing rusty or brownish areas on the surface.
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Hornblende – Dark green to black elongated grains that may be visible in some stones. Hornblende is harder than biotite but less reflective. It tends to form more angular or needle-like crystals and adds subtle variation to the overall texture and color of the stone.
Observable minerals.
Geological Significance
The size, color, and arrangement of these crystals tell a story of slow crystallization deep underground. Larger crystals indicate long cooling times, and the mix of light and dark minerals produces a speckled, “salt-and-pepper” appearance. Over time, some iron-bearing minerals may oxidize slightly, creating rusty or orange tints along edges or cracks. Observing these features helps you understand how different minerals form, coexist, and weather in crystalline rocks.
Example of on-site stone.
Tasks for This EarthCache
To log this EarthCache, visit the site, choose a stone to observe, and answer the following questions. Send your answers to me via Geocaching or email.
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Include “Mineral Analysis - Geocaching HQ - GCBHH2Q” on the first line of your message.
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Look closely at the milky-white crystals. Describe their color, shape, and abundance. Which mineral do you think they are?
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Find the dark, shiny flakes. How are they distributed across the stone? What mineral do you think this is?
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Which mineral appears most abundant in the stone? Describe its color and distribution.
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Observe the size of the crystals in the stone. Are most grains large, small, or varied in size? What does the size of the crystals suggest about how the minerals formed?
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In your log, attach a photo of yourself or a personal belonging with the Geocaching HQ sign on the side of the building in the background. This photo MUST be attached to your "Found It" log. (Note: photos predating the publication of this EarthCache are not accepted.)
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Works Cited
https://www.nps.gov/yose/learn/nature/granite.htm
https://www.geologyin.com/2024/06/granite-composition-and-colors.html
https://sites.northwestern.edu/sethstein/a-small-is-beautiful-approach-to-upgrading-a-beginning-geophysics-course/cooling-rate-and-crystal-size/