Etrog is a kind of lemon growing in Israel. It is one of the four staples of Sukkot (Feast of Booths). Sukkot lasts for seven days and pays tribute to the booths Israelites lived in during their 40 years of travel in the desert after the Exodus. "And you shall take on the 1-st day the fruit of beautiful trees (etrog), branches of palm trees and boughs of leafy trees and willows of the brook, and you shall rejoice before the Lord your God 7 days! Live in temporary shelters for seven days: All native-born Israelites are to live in such shelters." Leviticus 23:40 and 40:43
The fruit of the “beautiful trees” has always been in high esteem. It was a rare commodity and cost quite a lot. Therefore it was kept in special cases made of silver and white metal.