Bejeweled Seattle
If you've ever bought a ring, shopped for a necklace, or browsed a jewelers catalogue then you know that gemstones are an expensive commodity. Knowing how valuable gems are, you might be surprised to find some sitting out here in the open on Second Street in Downtown Seattle. Before you stands the Millennium Tower, which was built in 2001. For today's earthcache lesson we'll be examining the columns that intersect the sidewalk and looking at the gemstone embedded in the stone.
Please note that this is an earthcache! As such there is no container to find here, rather you will read the following geology lesson and make observations at the posted coordinates and send your answers to the Cache Owner in order to log your find! You do not need to enter the building or leave the sidewalk to log this cache (i.e. you don't need to block any doorways or ATMs while gathering your information).
January's Birthstone
It is fitting that this cache is being written in January, because the subject of today's lesson is Garnet! Garnet is a mineral that has long been used as a gemstone and as an abrasive as far back as the Bronze Age. Garnets are well known for their strong beautiful colors and high gloss. When polished they are often used in jewelery and are the official birthstone for individuals born in the month of January! While most people are familiar with their dark reddish brown coloration, depending on their chemical makeup they can also come in black, yellow, or even a ruddy green! In addition to their function as jewelery, garnets can also be used as an abrasive material due to its significant hardness of 6.5 to 7.5 on the Mohs Hardness scale* or 100 on the absolute hardness scale.
*There are two primary means by which geologists measure the "hardness" of a crystal: The Moh's hardness scale and Absolute Hardness. The Moh's hardness scale is a relative and somewhat random scale of scratch resistance, and the difference between the different levels can vary a lot from absolute hardness. Absolute hardness, however, is determined under controlled laboratory environments, but this scale is typically only used in academic environments.
Internal Structure
Garnet most often can be found in nature in a rhombic dodecahedron crystal structure, which is a fancy way of saying a 12-sided surface. This is because garnets typically crystallize into a cubic system, with three axes of equal lengths, all of which are perpendicular to each other. Because of this, garnets have no cleavage angle.
Garnets can be made of a wide variety of silicate minerals including magnesium (Mg2 +), iron (Fe2 +), manganese (Mn2 +), calcium (Ca2 +), aluminum (Al3 +) and chromium (Cr3 +). These minerals given garnets their wide variety of colors. In fact, there are more than twenty varieties of garnets, however the most common types are:
- Almandine garnets which are a deep red in color. This is created from a combination of Iron and Aluminum [Fe3Al2(SiO4)3].
- Grossular garnets which are on the darker color side of the spectrum forming garnets with deep greens and blacks. This is created from a combination of Calcium and Aluminum [Ca3Al2(SiO4)3]
Garnets occur naturally in metamorphic rocks such as marble, gneiss, and amphibolite, and are used as a baseline for geologists. By examining the garnets in rocks, geologists are able to determine what pressure and temperature the metamorphic rock formed in.
Logging this Cache
To log this cache you must answer the following questions and send your answers to the Cache Owner. You can do this via email or by using the message center feature on geocaching.com or in the Geocaching app. Please submit your answers in a timely fashion and feel free to reach out to the CO if you have any questions!
Questions:
1. Go to the given coordinates and examine a column in front of you. Embedded in the metamorphic rock you'll find several garnets. Which of the following units of measurement would be best used to accurately describe the size of these garnets: Inches, Millimeters, Feet, or Yards?
2. Look at the photo in the first section of this earthcache. Which option is most similar to the garnets that you find at the posted coordinates: A, B, C, D, or E?
3. What color are the garnets in front of you?
4. Based on this information would you classify the garnets here as Grossular or Almandine? Based on this what chemicals make up these garnets?
5. Please include a photo of yourself or a personal object at GZ. Do your best to ensure your photo doesn't reveal the answers to the above questions.
References:
- https://www.gia.edu/garnet-description#:~:text=There%20are%20more%20than%20twenty,crystals%20too%20small%20to%20cut.
- https://commonminerals.esci.umn.edu/minerals-g-m/garnet
- https://rocksminerals.flexiblelearning.auckland.ac.nz/minerals/garnet.html