JDman...
You are correct (from sandy rock or sediment) that was once on the ocean floor.
I first heard about it back in the mid 90's... while watching a TV program on Organic Gardening. They said that trace minerals are beneficial for improving the flavor of tomatoes. I just love tomatoes.. and if I can add a supplement that makes them taste better, all the better !
Below is some info I found on Greensand online... and a link to wikipedia that gives some good details.
There are various sources for it, some in the USA (like New Jersey) and others overseas.
TNhunter
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en.wikipedia.org/wiki/Greensand
What the Heck is GreenSand?
We get this question everyday and never grow weary answering it. The reason is simple we love Greensand. If you have never used the product (or never heard of it) Greensand is a mineral called glauconite, which is found on the ocean floor and mined for use as an organic fertilizer and soil conditioner. Greensand supplies marine potash, silica, iron oxide, magnesia, lime, phosphoric acid, and 22 trace minerals. Greensand works great on Southern “hard as a rock” clay soil this spring, to help improve its texture, and enrich the nutrient content of my soil without using chemicals. Greensand is one of the oldest and most generally useful tools in the organic gardening tool box. Greensand has been used since the eighteenth century as a soil amendment and slow-releasing fertilizer (0-0-3). It is one of the best certified organic sources of potassium. It comes from sandy rock or sediment containing a high percentage of the mineral glauconite (greenish-black to blue-green). It contains potash, magnesium, iron, silica and can contain as many as 30 other trace minerals. The recommended usage is to broadcast 50 to 100 lbs. per 1,000 square feet. It improves plant health in general and is also be used to loosen heavy, clay soils. It will also improve the moisture retention of sandy soils. Greensand is OMRI listed for organic use.