This may seem weird but.

Rocks should be fine, but your customs declaration will specifically ask if you have soil or dirt. It's your choice how you answer...

I've brought home small stones and tiny bits of dirt from Europe. I just carried it among my things in my suitcase. Nobody goes thru your things. I have no idea whether this is illegal or not. I suppose with the dirt you might import alien germs.

I looked this up once and discovered that it is not permitted to bring soil back into the US unless you have a special permit for it, although I think I remember that pure beach sand might be permitted. I think it would be best to search on the government's Customs and Border Protection website to make sure.

I remembered quite a long discussion about this back in April of this year. I think you will find your answer in that thread. I've had a look in Search and the thread was http://www.ricksteves.com/graffiti/helpline/index.cfm/rurl/topic/82612/sand-from-normandy-beaches--us-german-italian-uk-customs.html Sorry the link didn't work... Maybe you'd have a look there?

If you bring home a small stone you would be fine.

Bring back a small stone. We did this for our kids' pediatrician once. She loves to collect small pebbles/rocks from around the world. We brought her one from Mt. Caringorm in Scotland. We also brought back a pebble from Brighton Beach for a friend of my husband. They are both HUGE fans of Queen and they have a song titled "Brighton Rock". Never had any problems.

When you travel with small boys, many rocks find their way home in the luggage. Upon return from Africa, I found about 5 lb. of rocks in my son's backpack. No one seems to care and we've never had a problem--even told the customs guy at SeaTac about them and he just laughed.

No soil!!!! It is not allowed as it may carry pathogens. But small stones/rocks ( clean) are fine. I have several from my travels on my windowsill and in my garden.

I bring home rocks from all over. I have never been asked about them

Dont bring dirt from a farm- I think they still ask on USA rentry whether you visited a farm.

Get her a small stone from somewhere significant- eg from London, maybe from one of the parks, and tell her where you found it. While on a RS tour last year in Turkey I collected some pretty stones and bought some small purses to give as gifts to a couple of people I was on the tour with- they loved it.
Stones don't break any rules but soil does.

My wife picks up small stones and sand from all over the world. We carry tiny ziplock bags (purchased at Dollar Stores) for this purpose and she labels them along the way. As advised above, sand or stones(washed en route) should be fine. I wouldn't advise that you return with a large bag of fine, white sand - it might slow you down at customs!

Janis I brought back a chunk of 'COAL from Newcastle'. Don't remember how to story goes, but seemed a good souvenir. Also found a really neat piece of flint from Canterbury and are used to make walls,etc. I like souvenirs that don't cost a lot and have great memories. No question is really weird on Travelers Helpline.
Jana

I've brought back many rocks from the UK. It's no big deal at all, just give them a bath. I've also brought back shells, broken pottery, pipe stems, etc. and never any questions. Just don't bring back something of cultural significance. A student of ours got into MASSIVE trouble on a school trip by the fairly insignificant rock she picked up near the Parthenon. Not worth it! One of my prized possessions is a screw with paint on it from the National Theatre. That isn't too weird, right? Right?

I also I have brought back stones from all over the UK. And pieces of slate, pieces of seaglass and, well, other things. Another thing I do is to take along with me small envelopes. Along the way I'll pick a flower or a leaf, or some wool or horse hair from a fence ( your boss is not strange at all). I press the flower for a time in my travel journal before transferring it to an envelope with a date and location writtin on it. I only wish I could bottle smells.