Rockhounding in Delaware – What Rocks You Can Find and Where

Muscovite Mica

Streak: White

Hardness: 2 – 2.5

Chemical Formula: KAl₂(AlSi₃O₁₀)(F,OH)₂

Muscovite is the most common mineral of the mica family. It is an important rock-forming mineral present in igneous, metamorphic, and sedimentary rocks. Like other micas it readily cleaves into thin transparent sheets. Muscovite sheets have a pearly to vitreous luster on their surface. If they are held up to the light, they are transparent and nearly colorless, but most have a slight brown, yellow, green, or rose-color tint.

Photo: Muscovite Mica by James St John is Licensed Under the Creative Commons Attribution 2.0 Generic license.

Garnet (Almandine)

Streak: white

Hardness: 7 – 7.5

Chemical Formula: Fe₃Al₂(SiO₄)₃

Garnets are a group of silicate minerals. Almandine is the most common of the garnet group and always features a red color.

Crystals of almandine often have well-developed faces and complex crystalline structures. Due to its color, almandine garnets are frequently cut and used as gemstones, but coarse varieties are often crushed for use as an abrasive.

Photo: Almandine by Rock Currier is licensed under the Creative Commons Attribution 3.0 Unported license.

Quartz

Hardness: 7

Streak: White

Chemical Formula: SiO₂

Quartz is one of the most common minerals found in the Earth’s crust. While it is usually colorless and transparent, quartz can also be found in a wide variety of colors and a range of opacity. These varieties generally have their own name such as rose quartz, amethyst, smoky quartz, and citrine.

Quartz can also be found in cryptocrystalline varieties, or quartz that is made up of microscopic crystals. These varieties include chalcedony, agate, and jasper.

Gold

Streak: Golden, Yellow

Hardness: 2.5 – 3

Chemical Formula: Au

Native gold is an element and a mineral. It is highly prized by people because of its attractive color, its rarity, resistance to tarnish, and its many special properties – some of which are unique to gold. No other element has more uses than gold. All of these factors help support a price of gold that is higher than all but a few other metals.

Trace amounts of gold are found almost everywhere, but large deposits are found in only a few locations. Although there are about twenty different gold minerals, all of them are quite rare. Therefore, most gold found in nature is in the form of the native metal.

Photo: Gold by Robert M. Lavinsky is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Pyrite

Streak: Greenish-Black

Hardness: 6 – 6.5

Chemical Formula: FeS₂

Pyrite is the most abundant sulfide mineral. It’s pale brass-yellow color is where pyrite gets its nickname “Fool’s Gold” as it was commonly mistaken for gold by novice prospectors.

The name pyrite is derived from the Greek phrase pyrites lithos, “stone or mineral which strikes fire,” due to the fact that pyrite emits sparks when struck by iron.

Pyrite is well known for it’s cuboid crystal habit, but it can also be found in many other forms. Pyrite can form dodecahedral, or twelve sided, forms known as pyritohedra.

Fool’s gold can be differentiated from real gold by its hardness, brittleness, and crystal form. Pyrite’s streak also is a defining characteristic as it is generally greenish black to brownish black whereas gold tested on a streak plate will leave a yellow streak.

Sillimanite

Streak: White

Hardness: 7

Chemical Formula: Al₂OSiO₅

Sillimanite is an aluminosilicate mineral with the chemical formula Al₂SiO₅. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in Chester, US.