I am a U.S. citizen but I live in Mexico and I have on numerous occasions brought rock samples from Mexico into the U.S. without a problem although one time I had to give the U.S. custom officials a lengthy explanation of the significance of the samples not because there was a problem but because they simply were interested! I also brought sand samples from Australia into the U.S. via  San Francisco without a problem. I don't think you should have a problem unless your samples are gold, diamonds, or other than rocks that have no value except as "rocks". 

Many Thanks for the sharing the experience and the advice. 

Amar,

As someone who ships considerable amounts of material internationally - including radioactive samples - there are few regulations one has to abide by. Both the 49 CFR (Code of Federal Regulations) and IATA (International Air Transport Association) have no regulations that I know of related to rock samples.

As mentioned should you be questioned at Customs make sure that you tell any officer/official that your materials are geological rock specimens - emphasis on 'rock.' Do NOT identify them as soil samples. I would also mentioned that your samples are for 'research and/or scientific analysis'. Be prepared to give a basic overview of your work should they be curious.  And, as Kenneth Towe mentions above try not to be cute with your answers.

Best of luck.

http://www.ecfr.gov/cgi-bin/text-idx?tpl=/ecfrbrowse/Title49/49cfr173_main_02.tpl

http://www.iata.org/Pages/default.aspx

Dear Amar,

I feel you need to wash the samples, and put them in clean plastic sample bags, before packing and transporting through US customs and immigration.