3: Peculiar stones

Crocoite

The name comes from the Greek word krokos, which means “saffron”. The name refers to the bright colour of the mineral. Tasmania has significant crocoite deposits. The colour and the impressive crystals make crocoite a popular collector’s stone.

Pyrite and Marcasite

Pyrite is often found in small quantities at a time mixed with other rock types. Pyrite and gold form under similar conditions, and they often occur together in the same rocks. Sometimes there are small quantities of gold within pyrite, and therefore the most important use of pyrite is now as an ore of gold. Pyrite used to also be an important ore for the production of sulphur and sulphuric acid, but today, most sulphur is obtained as a by-product of oil and gas processing. Pyrite is occasionally used as a gemstone, and it was especially popular in the 19th century. The gemstones of this era were often mistakenly called marcasites, even though they were actually pyrite. Pyrite is not, however, a very good gemstone because it stains easily.

Real marcasite is a variant of pyrite, with a similar chemical composition but a different crystal structure. Marcasite does not occur in the same brass-yellow shade as pyrite, but it is lighter and sometimes even greenish. The crystal structure also makes it more brittle than pyrite. It is also slightly rarer. Marcasite may form rounded nodules and flat, disc-shaped concretions.

Imatra stone

“Imatra stone” (imatrankivi) is a rare type of stone, born in the Imatra Rapids and occurring exclusively there. The stone is composed of a core that spun in place in the clay deposit during the melting phase of the continental glacier for around two thousand years until the clay cemented around the stone. This gives the stone its round shape. The Imatra Rapids began to form after the glacial period around 6,000 years ago. They have remained there for thousands of years, which is rare on a global scale.