A clump of iron coins, seems like a mixture of mostly Southern Song coins with a few Northern Song coins. The coins are 2-cash or bigger. The clump is around 7 cm at its longest and weighs around 205 grams. You can see a bunch of coins stuck in it, along with some sediments. A very interesting clump - I think it can be used mostly as a decoration or conversation piece. Some of these are quite rare and fairly expensive, but the coins are very heavily encrusted and are nearly impossible to clean (at least I could not figure out a way to do it). I imagine cleaning the clump would be nearly impossible. I soaked it in oil for around 10 years, hoping to liberate the coins, but it does not seem to have helped.

The clump is reportedly from a South-East Asian find - these are often pulled up by fishermen in Indonesia, where this sort of coins were imported during the Song period. 

The Song Dynasty was a ruling dynasty in China between 960 and 1279; it succeeded the Five Dynasties and Ten Kingdoms Period, and was followed by the Yuan Dynasty. It was the first government in world history to issue banknotes or paper money, and the first Chinese government to establish a permanent standing navy. This dynasty also saw the first known use of gunpowder, as well as first discernment of true north using a compass. The Song Dynasty is divided into two distinct periods: the Northern Song and Southern Song. During the Northern Song, the Song capital was in the northern city of Bianjing (now Kaifeng) and the dynasty controlled most of inner China. The Southern Song refers to the period after the Song lost control of northern China to the Jin Dynasty. During this time, the Song court retreated south of the Yangtze River and established their capital at Linan (now Hangzhou). Although the Song Dynasty had lost control of the traditional birthplace of Chinese civilization along the Yellow River, the Song economy was not in ruins, as the Southern Song Empire contained 60 percent of population of China and a majority of the most productive agricultural land. The Southern Song dynasty was conquered by the Mongols led by the famous Kublai Khan in 1279.This coin is unconditionally guaranteed to be authentic.