Buoyancy - why and how.

In researching buoyancy, I looked a several sources on the web; wikipedia offered a lengthy involved discussion about columns of pressure in water and containers.  It was a little too involved for me to focus for long on, but I did get the gist of what was said.

Simpler explanations described buoyancy as a comparative relationship between the density of water to the density of a floating object.  The formula for determining the density of water is

mass of water / volume of container = density

When the density of the water is greater than the density of the object, the object will float.

Does this seem logical to me?  Not always.  Why don't more things float then?  Why won't a small pebble float instead of plop and sink?  I guess that one pertains more to the column explanation than the container explanation.  Also, I wonder, why if I crushed the pebble into dust, the dust flakes would float?  Same volume of material?  I guess the dust is not as dense although it would be the same volume, or would it?

Something I know about buoyancy and animal fur is that some animals fur or hair becomes more hollow in the winter in order to conserve body heat, therefore, the carcass of an animal will float on the water in the wintertime, but in the summertime it sinks.

Why does Ivory Soap float?  Does it really have to do with purity, or is that all marketing blather lather?  As I suspected, it  has nothing or very little to do with purity.  It floats because there are more air bubbles whipped into it during the manufacturing process.  Whether it has perfume or not has nothing to do or a negligible effect on whether or not the bar floats.  On the other hand, despite the fact that it has air in it, I've never really noticed that the Ivory bar doesn't last as long as other comparable bars, of course triple milled soap lasts longer than Ivory.  No bubbles, and lots of perfume.

Some things I would like to test for buoyance: sheets of wax, brass wire tied into a sort of netting, empty jars, full jars, a book maybe, cooking oil (why does an oil slick stay on the surface of the water when it's so dense and disgusting?).  I noticed alot of what looked like oil on the river last weekend, I wondered not only why it floats on top, but why it all seems to stay together in a puddle the way it does.