St. Petersburg, Russia
One of the most extravagant interior spaces is the amber room. The entire room is covered with panels of amber mosaic. The original panels were initially a gift for Peter the Great from a Russian craftsman. Unfortunately, the original panels were lost when German troops took over the palace and shipped them off to Konigsberg. In 1982 there was a restoration effort for the amber room, which took over 20 years and $12 million dollars to finally complete.
For more information click the link below for a video tour through the palace.