Wednesday, November 22, 2017

Thanksgiving with Dendrites



 I've always loved dendrites in my rocks. Dendrites are markings in stones that look like ferns or trees or other plants--if you catch them along their long direction. However, when you cross-cut them, you get a bunch of dots like the two different angles of the same piece of dendritic opal below.









In today's blog, I'm taking a strict interpretation of dendrites. Some people like to include plume agates with dendritic agates,but aside from the fact that they look very different to me, Roger Pabian, co-author of "Agates: Treasures of the Earth" has clarified the difference in that he states that Dendrites form inside existing agates and Plumes form first and then are surrounded by agate. 


  In this picture, the oval on the left is dendritic opal, the oval on the right is plume agate. Notice how the plumes look a bit fluffy?






It is further complicated by the fact that not all rocks with dendrites are agates. However, they usually are made of chalcedony, a term used to describe a group of microcrystalline silica-based minerals that include quartz, jasper, and onyx. They are great for lapidary work all coming in at 6.5 to 7 on the Mohs scale of hardness. Another mineral known for dramatic dendrites is Rhyolite which is also a silica-based mineral. It's not a chalcedony because Rhyolite crystals are much larger than the microscopic ones that define chalcedony.  Some of my favorite dendritic rhyolite is found in the Sonora area of Mexico (and rock shows!)

And then there's dendritic sandstone. Sandstone doesn't have a place on the Mohs scale because it can be so soft you can rub it off with your finger up to something very hard that can be used in construction.


From a mineral standpoint, dendrites consist of iron or manganese from nearby rocks that seep into cracks in the chalcedony and crystallize as they harden, creating the fern or tree patterns that make them so popular. In my readings about them, I was disappointed to see that they were nearly always described as black or brown in color, especially since I have two onyx rocks on my desk right now; one has dark blue dendrites and the other has dendrites the color of burgundy wine. It turns out that manganese oxide comes in a rainbow of colors including pink, violet, green and others.



Whenever people try to place walls around the natural processes, there are usually exceptions. This is a picture of a moss agate. Like other dendrites, it forms in the cracks of established chalcedony rocks. The difference is the mineral inclusion is green hornblende. It's as delightful to work as any of the others, and the startling green color is always an eye-catcher!




For all my readers in the USA, have a happy and blessed Thanksgiving.  As others begin to prepare for seasonal holidays, I wish you all the best.

Right now, I'm working on a blog post about how minerals get inside geodes--or when you see a channel between the interior of the geode and it's outside is the material expanding and flowing out, or is it the minerals lining the geode flowing in?

Your Lapidary Whisperer,

Donna Albrecht

Wednesday, November 8, 2017


 I had a blast last weekend!  It was the annual show for one of my rock clubs, the Contra Costa Mineral & Gem Society in Concord, CA Going to a show is a great time to shop, learn from displays and demonstrations, and talk with other rockhounds and lapidary artists.

I spent a lot of hours working the ticket desk, as I often do. It was really wonderful to see so many scouts and families with children! The little ones were so happy at the show and loved taking home their newly assembled rock collections, painted rocks,  and rock prizes from the "Wheel of Fortune" game (it's better than the tv show, everyone who played won!)


Another kids activity that was very popular and educational, gave kids the chance to learn about igneous, sedimentary, and metamorphic under the tutelage of an adult instructor. The best part was having the ability to handle examples of the different classifications.


Our club has a long history of displays in cases where our guests see great stone art, jewelry, and educational elements created by our members and some guests. In addition to the forty-seven display cases in the main rooms, we had special displays like the model railroad with all the buildings and landscaping in its environment made of stone and a special room with three cases of fluorescent minerals.

 Here's one I love, The images are all mineral intarsia and very detailed. Sorry about the flash spots, our cases each have two spotlights coming through the top.

 This is one of my cases. It's all about stem casts and shows three different styles, the basic petrified wood, opalized wood, and agatized wood. Because there isn't a lot of color, I took some pictures of a tree in front of my house when it was bare-branched, enlarged the heck out of it, printed it, and mounted it on cardboard cut to fit the three inside walls.


I LOVE shopping for rocks and slabs and I'd be happy if all our vendors specialized in rough and slabs, but in order to attract a wide variety of people (so we earn enough money to support the club programs and activities for the coming year) we have a range of vendors including fine jewelry (including pick your stone, pick your ring, bingo!) popular jewelry, specimens of all sorts, beads, small lapidary equipment and supplies, and even some novelty vendors like this one that painted messages on rocks of all sizes. We had twenty-five vendors, who each took multiple tables creating a lot of shopping fun.

 Lots of visitors were entranced by the meteorite vendor. His display was, to say the least, "Out of this World".

I think this vendor had every kind of 'findings' you could ever imagine! You not only can see and touch them to be sure they're what you want, you can purchase in small quantities--just the thing for the lapidary hobbyist!


 Fortunately for my budget, I was working so much that I had relatively little time to shop at the show. 

But sometimes, a little time is all it takes. :-)

Our society has a big table with silent auctions throughout the event. The rocks that are auctioned off are donated by members.

A lot of the material is rough rocks, usually with the name of the rock next to it.  A lot of the kids are especially interested in having a big rock and they'll bid on some. Sometimes rock hounds recognize a rock that has real potential to become one or more cabochons, and they'll bid.

As for me, I like the thrill of the hunt--as long as I don't have to bring a big rock down off a mountain. So when I saw this big beauty, (the yardstick shows it's nearly two feet long and it's nice and thick) sitting next to the auction table, I was intrigued. The label said agate.

Umm, maybe.

In the dark area of the upper left, there are what look like ends of square crystals. The coloring on the top appears to be weathering, with a few artistically placed blobs of bird poop. The bottom right now, is too dirty to see anything clearly. I'm hoping to get a chance on a big saw next week and check out the inside.  More on this later!

Hope you're enjoying the show season. It's just getting started and looks like it's going to be a great one. 

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Until next time,

Donna Albrecht
Your Lapidary Whisperer