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Rapakivi is a unique type of granite containing large crystals of orthoclase covered with a rock forming mineral known as oligoclase. It is also widely known as crumbly or rotten rock due to its unique characteristics of crumbling when rubbed with the bare hands. This phenomenon is widely due to the different layers within the rock structure. Rapakivi is a Finnish word which in means crumbly rock. The Rapakivi rock was first discovered by a Finnish by the name Serderholm Jakob and the rock is widely found in the Southern part of Finland.

Rapakivi granites, that have rounded phenocryst, which are about 1 to 5 cm alkali feldspar, which are entirely mantled by grey plagioclase, which is ultimately, an oligoclase. This formation provides what is known as rapakivi feldspar rapakivi texture. Thus, the rapakivi texture also refers to any granite stone characterized by the presence of oval grains that are usually mantled by plagioclase. They are plutonic rocks formed from the cooling and crystallization of molten rocks. The rapakivi is referred to as rotten because of its high vulnerability to weathering. The plagioclase feldspar is easily affected by weather conditions, and the unequal granulation of the internal structure of the rapakivi granite makes it easy to disintegrate due to temperature changes.

Rapakivi granites are extremely widely found in the south-west and south-east of Finland because these are the origin of it's first study but ever since the rapakivi granites have been sparsely located in other regions such as Brazil, South Greenland, parts of Sweden, Ukraine. The rapakivi is a tremendously useful stone in the decoration buildings.

The rock is widely quarried, and used in abundance within the building and decoration of churches, in the region of Southern Finland. Ultimately rapakivi was made hugely popular when it was used in the construction and design of some monumental buildings. Other significant uses of the Rapakivi granite are within railway construction, use in construction of bridges, use in construction of pavings and the decoration of walls.

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