Types of Rocks Found In Delaware: A Guide To The 9 Most Common You'll Spot

Though small, Delaware presents an awesome opportunity for collectors and amateur geologists alike.

In this article, you will learn about the most likely specimens you’ll look for in this state.

Types of Rocks Found In Delaware: A Guide

Introduction

We’ve all picked up rocks and wondered what they were and where they’ve come from.

But Delaware isn’t a state you’d usually consider a rich source of collectible rocks, as the state is known mostly for its more recent sedimentary rocks and fossils.

However, all is not lost, with the Piedmont region in the northern part of the state, in particular, being home to a wonderful selection of metamorphic rocks and minerals.

Even the beaches present a treasure trove of quartz or serpentine for the day-tripper, hiker, or camper to explore.

Here’s a list of the more common rocks you’ll find:

1. Gneiss

Gneiss is a foliated, metamorphic rock. Identification is made easier by gneiss’s bands of varying mineral compositions. Some of the bands (or lenses) contain granular minerals. These are mostly bound together in interlocking textures of feldspar and quartz grains.

As a metamorphized rock, Gneiss is transformed from shale, then into slate, followed by phyllite, then schist, and finally into gneiss.

Contractors use gneiss both as a crushed and a dimensional stone. So, it’s strong enough to be used in road construction, building site preparation, and landscaping projects. Sawn and sheared blocks and slabs can be shaped for use as paving and curbing stones as well as structural elements in construction projects.

Some gneiss is durable enough to perform well as dimensional stones. Large blocks of gneiss are cut into smaller blocks and slabs for the construction industry. They are then further processed into pavers for pathways, curbstones, and structural or decorative slabs for use in residential and commercial buildings.

You’re most likely to find Gneiss in quarries and river beds.

2. Schist

Schist is a foliated metamorphic rock with plate-shaped mineral grains, large enough to easily see.

Shales and mudstones are compressed, heated, and chemically altered to form schist. Minerals such as muscovite, biotite, and chlorite are created and can be seen together with quartz and feldspar within schist.

The Schist’s brittle structure isn’t very strong, so it isn’t suitable for construction.

Schist is usually found with different gemstones included in the structure, so it’s not uncommon to see garnets, for instance, embedded into the rock. This makes for very attractive specimens in a range of colors.

Quarries, mines, and river beds are your most likely hunting ground for schist.

3. Marble

Marble is another metamorphic rock, formed from limestone. This formation takes place when limestone is put under the immense pressure and heat of metamorphism. The mineral calcite is marble’s primary constituent. It also includes various quantities of quartz, micas, and iron oxides, together with pyrites and graphite.

It is a versatile and well-known crushed or dimensioned stone used in the construction of highways, railroad beds, building foundations, as well as decoratively.

You will usually find it in old quarries, mines, and along river beds.

Its distinctive whitish color with veins of various colors running through it makes it hard to miss.

4. Sillimanite

Sillimanite is Delaware’s official state mineral and is found in a variety of colors ranging from clear to gray-white, grey-blue with glassy crystals.

Sillimanite is an important raw material used in high alumina refractories in the iron and steel, petrochemical, electrical, cement, zinc and glass industries.

It is widespread, making it easy to find, especially in the schists of Appalachian Piedmont and in the boulders found at Brandywine Springs State Park, just north of Wilmington.

5. Garnet

Beautiful deep red garnets are found in Delaware. Referred to as the Piedmont garnet, you can find these 12-sided crystals in the northern areas of Delaware along river beds and in the Piedmont.

Despite being known for their use in jewelry, the ones you’re likely to find in Delaware are mostly deformed and fractured, so are not suitable for settings.

It is also used in industry as an abrasive.

You will most likely find them along river beds and in the quarries of Northern Delaware.

6. Quartzite

Quartzite is a glassy, transparent to translucent mineral which is extremely hard and fractures like glass. You can easily identify it as it is found in most Piedmont rocks as well as being present along the coast.

You can find colored inclusions of fuchsite (a green mica) giving quartzite specimens a delightful green coloring.

Quartzite can also reflect light, with a range of exquisite semi-transparent specimens ranging all the way to translucent examples producing aventurescence, a hard-to-miss glittering luster.

This makes quartzite a sought-after specimen, producing colors that range from pink to blue, orange, and yellow. It is a wonderful stone to tumble, producing lovely beads, small ornaments, and cabochons.

As well as being pretty, this stone is abrasive resistant and very strong, making it an ideal structural stone.

7. Muscovite Mica

The Woodland Quarry in Northern Delaware is known for its muscovite mica, which is found in the form of mica books.

Muscovite mica is an easy mineral to identify, with varieties being black, white, bronze, and green. Muscovite micas are found in all Piedmont rocks except for the Wilmington Complex gneisses.

The translucent properties of this beautiful mica make it an ideal filler and extender for paints and other surface preparations.

8. Feldspar

Exposed, weathered rock and granitic pegmatites are the home of Feldspar. It’s easy to spot as a whitish mineral or pink porcelain-like gravel. It is often found as broken rock, with beautiful rectangular-shaped, shiny flat surfaces.

Feldspar is mostly found in the quarries in the Red Clay Valley.

Crushed Feldspar is used in porcelain, china, and glaze production.

9. Serpentine

Serpentine is a greenish-yellow, soft rock that can also be brownish in color. A pocketknife can carve suitable soft specimens.

Slivers of serpentine are found predominantly along the seashore.

Used for its decorative characteristics, Serpentine is useful for decorating walls, tiles, shingles, and as a paint additive.

Tumbled examples make excellent specimens due to their rich coloring and ability to produce a bright, glass-like sheen.t

Collection of specimens is generally restricted in most areas, so joining a local rockhounding club can often allow you access to otherwise off-limits locations like mines and quarries that are privately owned.

It is also illegal to collect specimens in national parks and historical sites. So, you will have to obtain permission to collect samples, as even public land includes privately-owned mining claims.

I hope that you have found some useful information on Delaware’s rockhounding scene and that it will entice you to visit this beautiful part of the country.