I had the chance to hunt a huge gravel parking lot recently. It is crazy how compacted the ground gets. I tried a bunch of tools and what worked best in that situation was one of those weed-pullers. They look like a large screwdriver with a pointed, forked tip. Poke and pry. If the ground is *really* hard then try a small rock-hounder's hammer or mini-mattock.
I find alot of good coins in old driveways. I learned my lesson fast after scratching a nice IH, it's best to not dig straight for the coin. Working around it to loosen the rocks up then scoop them out by hand until you break that hard surface makes it easier. It takes time but you won't ruin your find.
You can't dig thru gravel. You use a tine {rake}. I use a 3 tine rake and claw out the gravel. It's like working soil with a lot of small rocks, just rake it out. The rake I use has a telescoping handle. I got it at Sears in the Garden tool dept. Frank
I use one of those forked end weed pullers as well in gravel. I loosen the gravel with it then use my trowel to remove the gravel. It works really well but once in awhile you dig through the gravel and hit blacktop below it. Then your pretty much screwed.