[The KG9 Kid]

"... If you pick up 300 million grains of sand off a beach, what are the odds of even one jewel-quality diamond being in there?"

Not a relative analogy, I don't think; A beach would be the last place to look for a natural diamond.

Might be an OK place to look for a lost diamond that fell off a wedding ring though.

300,000,000 grains of sand isn't much more than a bucketful anyway.

[RightWhale]

I was going to say grain of gold, but then I thought of Nome, Alaska, where the odds would be good.

But you could find a diamond jewel in a bucket of sand, and that could represent earth. We have found an earthlike planet! In fact, it's right here.