What is Marcasite?

Marcasite is a mineral formed from iron sulfate. It is occasionally called white pyrite, but is not pyrite, or fool’s gold, as it is sometimes called. To further confuse matters, most gems called marcasite, are in reality pyrite. True marcasite disintegrates quickly and does not keep well in gem form.

The non-gem marcasite is yellow and may exhibit some green tints. It may also have white or brown colorations. It is a relatively soft stone with a hardness of approximately 6-6.5 on the Moh’s Hardness scale. It forms into what are called coxcomb aggregates, where the crystals are separated into crystals that are similar in shape to the head of a top of a rooster’s head. Because of this special formation, marcasite is prized in collections.

Unfortunately, collected marcasite deteriorates gradually, possibly as a result of bacteria. Bacterial agents found abundantly in the air may facilitate the breakdown of iron sulfate. Eventually, a piece of marcasite will completely deteriorate into white powder. One decaying marcasite may affect other minerals close to it. Marcasite specimens tend to be separated to slow down decay of this delicate but attractive mineral.

Marcasite gems or pyrite are steeped in lore and tradition. They are thought to be able to discharge negative energy, improve one’s communication abilities, and protect those who perform dangerous jobs. Such lore tends to be the result of pagan traditions.

Miners, however, often viewed pyrite negatively since it had a deceptive appearance, which many mistook for gold. Many based their opinions on where to dig or pan based on pyrite, and too late learned that these stones were worthless. Thus it inherited the name fool’s gold.

Today marcasite/pyrite gems are of value because they are an attractive black that appears iridescent in the light. They are also very inexpensive, with loose stones of several carats usually less than one US dollar (USD). Those who make their own jewelry often enjoy using pyrite because of its lovely appearance and low cost.

Marcasite, chemically known as iron sulfide (FeS2), is a mineral that shares the same chemical formula as pyrite but has a different crystal structure. It is less stable and less common than pyrite. Marcasite is recognized by its pale brass-yellow color and a crystal structure that is typically tabular or pyramidal, unlike pyrite's cubic form.

Marcasite differs from pyrite in crystal structure and stability. Marcasite forms in orthorhombic crystals, while pyrite forms cubic crystals. Marcasite is also more brittle and prone to oxidation. When examining specimens, marcasite's crystals are generally smaller and not as well-formed as pyrite's, which are often shiny and well-defined.

Marcasite is typically found in sedimentary rocks such as limestone, shale, and coal beds, as well as in hydrothermal veins. It is often associated with other minerals like gypsum, galena, and sphalerite. Marcasite forms in low-temperature environments where sulfur is present, often as a replacement mineral in fossils or as concretions.

Despite its instability, marcasite has been used in jewelry since ancient times, prized for its metallic luster. However, it's actually pyrite that is commonly used in "marcasite" jewelry today due to its greater stability. The term "marcasite" in jewelry refers to the style, where small pieces of pyrite are set in silver to create a sparkling effect.

Marcasite itself is not typically sought after for its economic value, unlike pyrite, which can contain trace amounts of gold and is used in the production of sulfuric acid. However, marcasite's presence can indicate the potential for other valuable minerals in the area, such as lead or zinc ores, which can be economically significant.

Marcasite can be hazardous to collect and handle over time because it is prone to oxidation, which can lead to the mineral disintegrating and forming sulfuric acid. This process can cause damage to a mineral collection and potentially pose a risk to health if inhaled or touched. It's important to store marcasite specimens in low-humidity environments and handle them with care.