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Silicified Fossil Coral Tumbling

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digit

This post may hold the record for the longest setup time (unless you count the millions of years of "setup" that are the basis for most of the posts on this forum). The ultimate origin was when I first saw posts from Jim (coralhead) and John (Sacha) showing some stunningly gorgeous silicified fossil corals. This was a treasure to hunt for so unlike the black and gray shark teeth (and other fossils) I had been pulling out of the Peace River and I am always up for new experiences so I contacted the two of them through the forum. In addition to the incredibly encyclopedic knowledge brought to this forum by its members, the social aspect of being able to communicate with other members who share your interest should not be overlooked as another great benefit of TFF.

After some discussions about where and how this fossilized coral was found I soon learned that Jim was organizing a trip back in August 2014 for some friends from other mineral and rock tumbling forums who were coming in from out of state to collect some coral. We arrived in southern Georgia and my wife and I were able to meet up with Jim and John in person (two of the nicest guys you're ever likely to meet--a trait that I believe is shared by the vast majority of TFF members). Jim had his hands full organizing the larger group that was coming in from various states to the north so John took us under his wing and Tammy and I were introduced to coral collecting. To call it "hunting" is a bit misleading as the bed of the Withlacoochee River is quite literally paved with chunks of fossilized coral--"shopping" would be a more apt term for what we did. The trick of course is to find some nice pieces where the calcium carbonate (aragonite) coral skeleton has been replaced over time with silicon dioxide as water has picked up this mineral from the silica rich sands and percolated through the corals to slowly transform the chalky white corals to a lustrous glassy chert. A quick strike on a corner with a rock hammer would usually open up a "window" so that we could see what the inner state of the corals looked like within their rocky (and sometimes algae covered) crusts.

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To say that we had a great time would be an understatement. As you can see from the photo above collected quite a bit--sometimes a bit indiscriminately as we were still novices and did not have a fine tuned eye for what would be a nice looking specimen. One of the goals was to find some pieces of coral that would (though transformed into silica-based chert over the eons) still show some signs of the original coral polyp structure. My wife has a favorite fossil coral pendant she bought in Bali several years ago and we thought it would be fun to try to find something like this ourselves. Unfortunately, our desire to aim for pieces retaining the polyp structure often led us to keep pieces that turned out to be "punky"--where the silica had not entirely replaced the calcium carbonate skeleton. These pieces (while displaying the polyps) were not glassy enough to take cutting and polishing or rock tumbling and have now become "yard rocks" in the back yard.

If you missed reading about this outing, check out this post from shortly after our trip (with lots of pretty images): http://www.thefossilforum.com/index.php?/topic/48828-first-coral-hunt/

-Ken

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caldigger

Thanks for the update, looks like some pretty stuff.

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digit

John has gracious enough to slice through some of the more promising looking corals with his slab saw (a tool prohibitively expensive to rank novice beginners). I'll have to add some photos of the nicer sliced corals soon. It turns out that hollow corals provided some of the nicer sliced specimens. These "hollows" were coral chunks that formed a silicified crust on the outside but then somehow "rotted out" from the inside leaving a hollow cavity aka "vug". These cavities were then coated with fine layers of silica and any small point on the inner surface of these hollows soon built up enough layers to form rounded "pearl-like" spheres known as botryoidal (grape-like) crystals. Many of the solid pieces that looked like they would not be particularly exciting when sliced were destined for another fate--the hammer.

Following Jim's advice I bought myself a rock tumbler and then systematically disassembled the larger chunks of chert I had collected into smaller flakes. I'd be lying if I didn't admit to a certain odd kinship to the Native Americans who flaked this same chert to make projectile points, knives and other sharp edge tools. My intention, however, was to create something as far from that as possible--something with smooth surfaces. Initially, I had been beating and bruising these pieces of chert with a larger sledge hammer which resulted in a lot of shattered shards and more than my share of scalpel like chips. These tiny blades cut such fine slices in arms, ankles, and any unprotected skin and continued to bleed for some time. I soon learned to respect this glassy material and not try to "make little ones out of big ones" without the proper protective attire. Jim explained that it is much better to remove flakes from a larger core which resulted in nicer shapes which had much less of the internal cracks and fissures that I had been generating with my brute force method. I found that chipping in my garage within the confines of a large cardboard box was one way of containing the glassy shards released while flaking. In a few days I had worked my way through my inventory of corals. I had weeded out the punky pieces and reduced the nice solid ones to smaller chunks for tumbling.

-Ken

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digit

After a little online shopping and with Jim's blessing, I selected an appropriate tumbler for my needs. As I had accumulated a fair amount of material to be tumbled I was not going to get a smaller capacity tumbler (like the one I had some 4 decades earlier when I last tumbled rocks). This one had a healthy 15 pound capacity and was a good heavy duty model (important when they run for days on end and for months to complete a batch). Jim also guided me to sources for good quality grits that would be essential for shaping and polishing my pieces.

My tumbler was shipped to my house and quickly assembled (few moving pieces as tumblers are far from high tech devices) Also arriving in the post were were the grits. I ended up getting a 5-gallon bucket of the most coarse silicon carbide (SiC) grit that I would be using the most of during the transformation from sharp shards to sensuously smooth stones. I had all the ingredients and I was ready to start making some noise in the garage.

-Ken

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digit

There was nothing left to do now but to load the hopper with a selection of the chipped chert. I loaded it about three quarters full (the suggested amount for optimal tumbling). A scoop of silvery SiC grit and just the right amount of water were added and the lid was screwed in place (with a rubber sheet protecting the lid and the stones within from meeting each other). Then all I had to do was load the hopper on the rollers of the tumbler's base and plug it in.

-Ken

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digit

Tick, tick tick...time s...l...o..w...l...y... passes by as the tumbling and grumbling and churning and turning noises continue to emanate from a lower shelf in my garage. After about 6 or 7 days it was time to pull the plug and open up the hopper to see how things were progressing. The pieces were starting to lose their sharp edges and were beginning their journey toward their desired shapes. The SiC grit had basically been broken down into nothing but a grayish slurry at this point and required topping off with another cup of fresh grit. This would be repeated every week or so for nearly 3 months. This process took a bit longer than it should have as I was often out of the country for extended periods and couldn't tend to the hopper and refresh the grit as regularly as I would have liked.

-Ken

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digit

After about a month and a half in the tumbler the pieces were starting to shape up nicely and I decided to toss in a few pieces of some of the smaller hollow corals that I had broken up. I wanted to wear down through some of the layers of the botryoidal "bubbles" but not to erase the texture altogether so I figured I'd add these near the halfway point during the shaping phase with the coarse SiC grit. Over time the pieces reached what I decided was their final shape. There were still rough spots on some of the pieces where there were still limestone inclusions or other patches of the outer crust but I specifically left the crust on these pieces from the start as I thought I might like this to add some interest to the finished pieces.

-Ken

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digit

After the shaping phase with the coarse grit was complete I switched to ever finer grits through a few pre-polishing stages before finishing with a very fine white aluminum oxide polish. I pulled the plug on the tumbler for the last time this morning. I pulled out the resultant pieces and after a good cleaning dried them out on a towel to inspect their finish. I still had several pieces which had fractures in them as the result of my clumsy initial efforts of busting up the larger chunks. I like the white inclusions on several of the pieces and though it detracts from the silky smooth finish I think it adds a bit of character.

As you continue shaping by wearing down the pieces with coarse SiC grit the volume of the rocks you are tumbling necessarily goes down (as the removed material is pulverized into a thick slurry). Toward the end I had removed so much material that the hopper was far from full and some of the pieces experienced "bruising" or chipping along thinner edges. I think I'll try to avoid this on my next batch by tumbling two batches (separately) for the first month or so and then adding them together once their bulk is reduced so that I have a relatively full hopper toward the end of the shaping phase.

I'm pretty happy with the results of my first batch of tumbled rocks since I was 10 or 11 years old. I've ended up with some pretty pieces and, more importantly, learned some lessons which I can apply toward future batches. Here is a display of some of the resulting pieces displayed in a flattened display bowl that I turned on my lathe several years ago from a slab of curly maple.

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I was reasonably happy with the botryoidal pieces I added to the mix. They ended up showing some really interesting patterns. I think further experimentation along these lines will turn out some really pretty pieces over time.

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-Ken

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digit

As creating jewelry quality pieces showing the coral polyp structure was a goal that originally spurred us to collecting silicified fossil coral, I kept my eye out for pieces that still had remnants of the original coral pattern. In many of the pieces the structure was lost and only pretty shades of milky white, rusty brown or red remained in the finished pieces but a few showed promise. Though not as boldly contrasting as my wife's pendant from Bali, some pieces did retain some polyp structure. As my wife succinctly stated, it doesn't count if you have to look under a bright light and magnification. Pretty examples of fossilized coral from millions of years ago? Yes, definitely. Ready to be made into stunning jewelry? Uh, still working on that....but having a heck of a lot of fun in the process.

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Cheers.

-Ken

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Sacha

Nice job Ken. The sell plastic pellets by the pound which can be added to the barrel during the later stages of the process. This prevents the bruising and enables better movement in the barrel as it rotates.

I think Jim told you to add more coarse grit to the barrel after the first week and subsequent weeks until you achieve the rounded shape you prefer. I think this delays the process. I run 80 grit for a week. Clean the barrel and rocks completely, then add fresh water and fresh grit. I run coral for 2 weeks and get the shape I like.

I have a 6 lb and a 12 lb tumbler and when polishing coral lately, run for a week in each, then add them together in the 12 lb barrel for a week. That starts me off with a nice full and solid load.

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digit

Great ideas. As I only have a single tumbler I'll run two different batches in succession and then combine them toward the end. I only started to get bruising once the barrel emptied out toward the end so I think if I keep it adequately filled the bruising should be minimized (I hope).

-Ken

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snolly50

Ken, I very much enjoyed the instructive, detailed account. Thanks for sharing the experience.

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ZiggieCie

Beautiful Gems and a great report. :1-SlapHands_zpsbb015b76:

For anyone interrested Harbor Frt has tumblers and grit.

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digit

Ken, I very much enjoyed the instructive, detailed account. Thanks for sharing the experience.

I'm all about experiences--both having them and sharing them. Glad you enjoyed my illustrated documentary on smoothing the rough.

-Ken

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Mike Pocock

Very nice I spend hours polishing fossils the old fashion way with wet and dry paper of different grades and finish off with aluminium oxide and a piece of leather.

Gets me a good result but I think I need to mechanise the process.

Thanks for sharing

Regards

Mike

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goatinformationist

Hey Ken,

Well being the cheapskate I find that a first week in the barrel with only water and each other to grind on does the same sort of job that adding in 80 grit does.  Use 80 grit in the second week and the week after - after cleaning the barrel and adding fresh grit and water - and the results will be stunning.

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digit

Interesting idea. I'll have to try that with my next batch.

 

Currently down in Australia. Visited a beach were we were viewing Fairy Blue Penguins last night. The entire beach is full of colorful cobbles instead of sand. Took some photos but later thought that I should have picked a bunch to finish off in the rock tumbler at home to see if I could put a shine to them.

 

Cheers.

 

-Ken

 

IMG_7641.jpg

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Sacha
On 10/24/2016 at 9:03 PM, goatinformationist said:

 

Well being the cheapskate I find that a first week in the barrel with only water and each other to grind on does the same sort of job that adding in 80 grit does.  Use 80 grit in the second week and the week after - after cleaning the barrel and adding fresh grit and water - and the results will be stunning.

 

I thought this was an great idea. I had a bunch of coral left overs that I didn't think were interesting enough to waste the grit to tumble them, but figured I'd give them a week or 2 of free churning to see if they might turn into something worthwhile. I filled my 12 pound barrel right after reading your post and I'm real interested to see what happens by next Tuesday. Most of my pieces are egg size or bigger. I don't know if that will help or hurt.

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Shamalama
23 hours ago, digit said:

Interesting idea. I'll have to try that with my next batch.

 

Currently down in Australia. Visited a beach were we were viewing Fairy Blue Penguins last night. The entire beach is full of colorful cobbles instead of sand. Took some photos but later thought that I should have picked a bunch to finish off in the rock tumbler at home to see if I could put a shine to them.

 

Cheers.

 

-Ken

 

IMG_7641.jpg

 

Man o Man, those are some lovely pebbles. Put a bucket of them up on the auction. ;)

 

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digit

My luggage is already teetering dangerously close to the 50 pound limit. I'd love to send home a suitcase full of those cobbles but 50 pounds wouldn't fill a carry-on. I know because I've filled a carry-on with rock before (Green River fish plates) and it comes out to around 70-80 pounds. :blink:

 

A few pretty ones did manage to find their way into my wife's suitcase though. We're off to Coober Pedy in a few minutes and we hope to noodle for a bit of opal (but likely will only find potch).

 

 

Cheers.

 

-Ken

 

P.S.: John, looking forward to seeing what comes out of your tumbler. I tumbled a couple of the thicker slices you made for me last year and it knocked the corners off and made them really nice.

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Jdeutsch

http://andy321.proboards.com/  is a web site that is dedicated to lapidary, tumbling,  etc.  They have some very experienced folks- It's a good place for technical questions when working with lapidary material

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sdsnl

3 months is a very long time! Is it not possible to polish with rotating sand paper? I have a little device like a dremel, but instead of moving up and down it rotates horizontally, and instead of the drill but you can insert a little wheel with sand paper on it. If the rock is not too hard and the number is not large this would get the polishing done faster, and you can have more control over which parts to polish.

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ynot
12 hours ago, sdsnl said:

3 months is a very long time! Is it not possible to polish with rotating sand paper? I have a little device like a dremel, but instead of moving up and down it rotates horizontally, and instead of the drill but you can insert a little wheel with sand paper on it. If the rock is not too hard and the number is not large this would get the polishing done faster, and you can have more control over which parts to polish.

If You use this method You will need a coolant on the rock or it can overheat and break.

 

Tony

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sdsnl
27 minutes ago, ynot said:

If You use this method You will need a coolant on the rock or it can overheat and break.

 

Tony

Put it in the fridge (above freezing) beforehand?

 

Does agate heat easier than other rocks?

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ynot
2 hours ago, sdsnl said:

Put it in the fridge (above freezing) beforehand?

Will only work for a couple of seconds before the stone will over heat.

 

2 hours ago, sdsnl said:

Does agate heat easier than other rocks?

The heat built up by grinding does not disperse through the (agate) stone easily. This uneven heating causes the hot area to expand more than the rest of the rock -- which causes the rock to crack. Water is used to draw off the heat before it can cause the stone to break. I used to "dip" my rocks in cool water repeatedly to keep it cool while making cabochons with an old grinding wheel. Very cumbersome, but it did work.

Different stones will disperse the heat differently.

Tony

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