Tag Archives: UP Thomsonite

Prehnite Adventure

Monday, August 8, 2011

When you’re rock hunting you don’t always know how good the pieces are that you have found. But I spotted a promising prehnite piece today while “driving by”, that I think might be a highlight. Here’s what’s been happening.

The famous (or infamous) lift bridge separating Houghton and Hancock.

The famous (or infamous) lift bridge separating Houghton and Hancock.

We arrived and got settled at the Houghton Campground last Thursday. The campground that the City of Houghton owns has 22 campsites for non-tent camping located right on the channel. This is a very scenic area and it is wonderfully tranquil with the occasional boat or jet ski traveling by. The channel bisects the Keweenaw Peninsula and is a shortcut from the east side of the peninsula to the west side.

Ships can pass thru the channel and when they do the lift bridge is hoisted, causing what the locals refer to as a “traffic jam”. We have been here when the lift bridge got stuck open and it really causes havoc. There is no other way to the north and the hospital is on the Hancock side, so if there is an emergency on the Houghton side (South) there is no way to get across except by boat.

U.P. Thomsonite is really ugly, but surprises await inside the ugly shell.

U.P. Thomsonite is really ugly, but surprises await inside the ugly shell.

Today we headed out seeking U.P Thomsonite (which is pink and green Prehnite). This makes wonderful jewelry and often shows chatoyant “eyes“. When you find the stones, they are an ugly brown or bleached white and you cannot know what is inside until you cut the rind off. Picking up the right white/brown stones is the tricky part! Lots of calcite and granite show up as little white stones, but they are not of interest and I had to keep throwing them out of Bonnie’s bucket!

I'm looking for the eroded out Thomsonite.

I'm looking for the eroded out Thomsonite.

We were with our friends Chris and Crystal. Prehnite fills voids in basaltic material, that over the years rots away and the Prehnite nodules drop out. You can dig for them in the rotted basalt, or sometimes find an eroded bank where you can simply pick them up. We filled a small bucket in short order from mostly eroded stones. We said goodbye to Chris and Crystal and decided we were done hunting for the day.

But, before we went back to the RV, I decided to explore an old logging road. I poked my head out the window, hoping to see a large agate. Chris got a big one today, and I was hoping to catch up. I saw a large seamed rock in the road and dug it up. A fantastic large seam of eyed Prehnite appeared in the rock. This was the find of the day and I think it will make some great jewelry.

A wonderful Prehnite seam.

A wonderful Prehnite seam.

While hunting today we found good patches of wild raspberries and Thimbleberries. No bears were about so we ate our fill. YUM!

We have not decided what is on tomorrow’s agenda yet, but we will keep blogging on our adventures here in the Keweenaw. The weather here has been ideal in the mid-70’s, but our best hunting is in the rain, so we are hoping for some soon. It has been very dry here all summer, but there was a nice shower the other night. That’s all for now.

Thomsonite and Prehnite

Rockhounding the the Keweenaw Peninsula of Michigan is one of my summer highlights. A distinctive set of stones are found there, many related to the presence of native copper. In this article I would like to talk about thomsonite and prehnite which are different stones, but sometime share the same name.

True Thomsonite may occur in the Keweenaw Peninsula of Michigan, but is generally not associated with copper.

Thomsonite is a zeolite akin to minerals such as tanzanite. pb2303101Thomsonite is a lovely pink stone sometimes featuring “eyes” and chatoyant sprays. The very best has always been associated with a small area near Grand Marais, Minnesota along the north shore of Lake Superior. The collecting area in that location has been closed to the public for many years, but Thomsonite pebbles still roll up on the beach near Grand Marais. The difficulty lies in most of the beachfront property being private property.

thomsonite

Most of what we use to refer to as Thomsonite from the Upper Peninsula is really copper-bearing Prehnite, or what is sometimes referred to as Patricianite. If you find what you believe to be Thomsonite and it contains specks of copper, it probably is Prehnite. The inclusions in the Prehnite are endless and create lovely pastel colors. Often “eyes” are present as are variegated type feathery patterns. We refer to these gemstones on our website as U.P. Thomsonite (this is what the locals call them also).

U.P. Thomsonite, in my opinion, is the most remarkable gemstone found in the Keweenaw. The striking chatoyancy and pastel colorations of this stone are something to behold. It is impossible to photograph the chatoyancy of true Thomsonite and U.P. Thomsonite as you really need to move the stones back and forth to see this effect. Thomsonite is much more expensive and scarce than Prehnite, but many people find the U.P. Thomsonite more attractive.

p9060246 Good, jewelry-grade Patricianite is still hard to come by. It took me ten years to locate a spot to dig this wonderful stone. The locals will not easily give up their best rock hunting locations, so you may be on your own when it comes to finding U.P. Thomsonite.

Our website features some of the finest Thomsonite and U.P. Thomsonite available.