Some 26,000 diamonds, worth at least $162,000 and contained in transparent plastic bags in hand luggage, proved too much for the "nothing to declare" lane at Sheremetyevo Airport.
An unemployed, 37-year-old Russian man carrying the precious package from Dubai was detained Sunday, a Federal Customs Service spokeswoman said.
"One customs officer noticed he was very nervous during regular luggage checks and was pressing a bag under his arm, so she asked him to let her see what was inside," spokeswoman Natalya Karaseva said.
The man, from the restive Dagestan republic in the North Caucasus, said he was carrying the diamonds as a favor for a friend. It was not clear whether he would face criminal charges.