Amber

The Amber is a standalone fossil that can be donated to the museum in all games to date.

In Animal Crossing
"Hoo! Again, I say hoo! Just look at this. Amber. Stoutly preserving the pale reminders of our past. And this is quite a large, full-bodied specimen, eh wot? Smooth. Exquisite. I'm reeling in awe! Amber is a form of tree resin, which has hardened and been preserved in the earth's crust for millions of years. Resin is produced as a defense against insects and disease. It seals wounds, allowing trees time to heal. Sometimes, unlucky insects get caught in the resin, too. I wonder what's in your amber? A fly? Perhaps a mosquito? Amber, you see, is a preservative of miraculous proficiency. Bits of amber are like miniature time capsules. Hoo, mercy! What prattling! I must beg your pardon. I imagine you feel as though you've been captured in time!"

In Wild World
The Amber in Wild World is rectangular.

"Amber is created when the sap from ancient trees hardens, wot? It does bear mentioning... Amber in and of itself is glorious. But if there is a bug trapped inside, well, then it is even more beautiful! The bug even retains the exact shocked expression it had when it was trapped!"

In City Folk
"Amber comes from the sap of ancient trees that has hardened over time, wot? Sometimes there are even insects caught within! Serves them right. It's beautiful, of course, and scientists are also quite keen on it, so it's rather expensive! These marvelous little fellows provide us with clues to the period when dinosaurs ruled the earth!"

In New Leaf
"Amber is simply fossilized tree sap, formed and hardened over many thousands of years. Prized for its beauty, amber is often used in accessories such as necklaces and earrings. Amber containing a trapped insect, such as a bee or ant, can fetch a particularly high price."

Value
Amber is very common to find, making it not very valuable. Villagers who have hobbies of fossil digging will accept it and probably store at their house. Though they aren't very proud of it, it will remain there until you send them or give them something better.