if it's vintage/antique ivory, you have to prove it. you have to be able to prove the date the ivory was harvested. in the piano instance, the date of manufacture, which is easily ascertainable, and proof the keys are original or pre-date the cutoff date in the statute is all you would need. most antiques dealers have affidavits and documentation proving any ivory they sell is exempt from the law. regarding ivory obtained from a foreign country, you have to prove the ivory isn't taken from archaeological sites, isn't protected by the laws of that country, or is not a national treasure which if removed from the country of origin would constitute, essentially, theft of art or national heritage. with specific regard to mammoth ivory, you have to be able to prove when it was discovered, where it came from and you might even run into problems w/regards to russian ivory of claims that it is the property of indigenous peoples. regardless of the importability of this ivory nut, failure to declare that the nut is ivory on any customs paperwork would be a separate, serious federal offense, the prosecution of which would really not have anything to do with the provenance of the ivory itself.
