December 2-9, 2006: Tampa Bay, Florida, Agate

One of many trip reports by Alan Silverstein.
Last update: March 6, 2008

From:  Alan Silverstein <ajs@frii.com>
Date:  12 Dec 2006 17:54:52 -0700
Subject:  collecting agate -- in Florida
To:  rockhounds@lists.drizzle.com

Last week in Florida I collected some agate and jasper... In fact I brought back over 50 pounds of it in my suitcase. Well OK, in both of my suitcases, after I visited Goodwill and bought another one for $3. And some of the material went into my wife's suitcase too, I had to distribute the weight to stay legal.

Anyway I can hear you muttering, "What? Florida is nothing but coral rock." That's what I thought too. I lived in the state for a little while back in the 1970s, and have visited my parents in the Tampa area many times since. Done a little snorkeling, a fair bit of shell collecting, but never any rocks... Other than the rare opportunity to visit a phosphate mine once.

I'd heard of Tampa Bay agatized coral geodes, but also that they were pretty much worked out. Last May I saw a specimen in the Smithsonian and it rekindled my interest. I did some research before our recent vacation trip. Thanks to the kindly folks at the Florida Geological Survey, I learned about Ballast Point Park, which is on the east side of the penninsula south of Tampa (between Old Tampa and Hillsborough Bays). I also got a pointer to the Dunedin Causeway, which goes west to Honeymoon Island (on the gulf coast north and west of Tampa). And they mentioned this excellent web article.

Some of you might recall that my main interest is tumbling material. I figured, where there were geodes, there might still be chunks of agate and jasper -- and indeed there were. I found it on the south side of the causeway west of the drawbridge, also all over the beach at Honeymoon Island (funny that I'd never noticed it before), and here and there at Ballast Point too. But I didn't spend much time at the last location, just passed through ten minutes before sunset, in a cold howling wind, to check it over... More on that later.

On the Dunedin causeway I found mostly small scraps of flinty gray to greenish or black agate in a calcareous matrix, in the water or at the waterline, with a lot of white shelly shapes and/or small drusy pockets or banding. It looked like the dredge rocks used to build the causeway have recemented pretty tough, but chunks are weathering out in spots, up to several pounds.

At Honeymoon Island we walked north from the north pavilion over 1.5 miles and back, mostly to shell hunt and enjoy the white sandy beach experience. We even saw a pod of dolphins (maybe 10 total) swimming along off the coast. To my surprise I noted a great deal of the same silicified stuff that I hadn't particularly realized in years past, very similar in variety and size (from chips to multi-pound boulders), at many spots on the shoreline. So I guess it's native to at least that area, and weathering out.

Washing and sorting the material, it ranges from occasionally quite interesting -- even some small drusy or botryoidal pockets -- to mostly boring -- crumbly coral rock, although full of holes, but not silicified after all. Also some of it is black inside a bright orange rather than white matrix ("limonite"?) The agatized portions grade quite irregularly into limestone... I found one small scallop shell, still with original color, embedded an inch or two from solid black opaque agate in a single chunk... Weird.

The rocks are surprisingly tough to crack with a hammer, whether or not silicified. I think the agate/jasper is fairly competent and solid, while the limestone absorbs the energy, especially when wet.

I found few small bits reminiscent of the inside of a geode, including a little blue/white "stalactite" half an inch long, but of course nothing like the specimen I saw in the Smithsonian in May (grin).

None of the rocks I found were well-shaped geodes, but very randomly silicified. Also not very colorful, mostly gray to black, some white or blue, but pale. However, some have interesting shell or coral shapes embedded in the agate/jasper.

Most of what I picked up to take home wouldn't be worthwhile if I found it, say, in Wyoming, but it's pretty special because of where it comes from. Until I tumble a load I won't know how it turns out, but it looks promising to at least be interesting.

Rockhunting along a shoreline, up to knee-deep in the salt water, sure was different than doing it in the western US deserts. It was the ickiest collecting I've ever done. It was often mucky and overgrown with moss-like coatings on the rocks, even muddy. Especially at Ballast Point there was a lot of trash on the "beach", and I couldn't tell what was inside any rock unless I cracked it open (goosh/splat) or chipped off a corner. Half of the rocks turned out to be concrete or bricks.

It's quite possible I stepped over a geode without knowing it, but any vug open to the outside was full of living things (shells and creepy-crawlies like worms) as well as thick mud and vegetation.