My friend's father is a buyer for a good size company, in his travels he comes across alot of this stuff. He always declares any items he bought when leaving a country to avoid any problems. From what I have heard from him, It is not illegal to own artifacts from other countries, It has to be a legally purchased item. There is tons of Japanese items floating around that were trophies of the second world war, legal to own yes unless there is a record on the books that the item was stolen.(ie Nazi seized art work) Definity harder to prove with gold or other non traceable items.
If the item was not illegally removed from the country of origin (stolen) or a national treasure than there aren't many restrictions on owning it.
China and some Asian countries prohibit removing artifacts from their borders due to rampant theft and a big black market demand. Other countries may have similar restrictions like in Brazil with the ban on exporting Rosewood or Gold. It is easy to find out importation legality by just checking with the Country's embassy or U.S. customs.
JerrY