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Alluvial Deposits

Derived from the Latin word “alluere”, “to wash against”, alluvial deposits are often a combination of soils, sediments, stones and minerals that have come to rest in historic river beds.


The amount of solid matter carried by a large river can be enormous: the Amazon River is reported to relocate over 700 million tonnes of sediment and rocks to the sea every year!

When exposed rocks at the surface of the Earth weather over a period of time, gems may be released. Some of these rocks will dissolve completely, while others will be broken down into smaller pieces. The smaller, loose rocks that survive the erosion process are often washed into rivers or streams and as gems are heavier than most other materials they can easily be trapped in depressions in stream or river beds; this is how concentrations of gems are found.

Over millions of years these rivers dry up and alluvial deposits can be found many hundreds of miles from the nearest river or sea. Gems that are mined from alluvial deposits tend to be rounded, due to the fact they have been rolling along river or sea beds.

As landscapes continue to change over millions of years, many alluvial deposits end up being buried deep underground. What I find most interesting about alluvial deposit mining, is that of the dozens of mines of this type I have visited around the world, no two have used the same method to search and extract the rough material. Whilst small, independent artisanal miners may dig deep shafts and then have their neighbouring miner lower them down their hole, big corporate companies will use heavy plant machinery.

When an alluvial deposit is mined, it can often yield a wide variety of different gemstones. Madagascar, which currently has many alluvial mining activities, boasts one of the widest arrays of beautiful coloured gemstones in the world.

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Local panning for gems in Ilakaka, Madagascar.

Entrepreneurial Sapphire miner in Sri Lanka

searching an alluvial deposit just below the

ground.

Me searching for Morganite in Madagascar.

The deposit is some 30 metres down this shaft.