Different Directions

Geodes are proof of that because you can't judge them by their coverings. From the outside, a Geode looks like an ordinary, slightly rounded rock.

And they have irregular surfaces of limestone, or other sedimentary or volcanic rock.

Well, the term geode is derived from the Greek word γεώδης - geōdēs, which means "earthlike."

Geodes are essentially rock cavities or vugs (see below) with internal crystal formations or concentric banding. The exterior of the most common geodes is generally limestone or a related rock, while the interior contains quartz crystals or chalcedony deposits or both.

When geodes are completely filled with crystal or chalcedony (see below), being solid all the way through, they are called nodules.

The only difference between a geode and a nodule is that a geode has a hollow cavity, and a nodule is solid.

How Are Geodes Formed

Geodes need just the right combination of water, chemicals, and pressure to produce crystals – plus a cavity in which to grow them.

Cavity

Geodes begin as bubbles in volcanic rock, or animal burrows, tree roots, or mud balls in sedimentary rock. Over time, the outer shell of the hollow shape hardens, and water saturated with minerals slowly seeps into the cavity.

The minerals that make up the water/mineral solution fall out of the water and coat the inside of the hollow. This process is called precipitation, and is like snow falling out of the sky and covering the ground – or in this case lining the inside of the geode cavity.

This “snow” is usually a silica precipitation on the inside walls of the cavity within the geode. This precipitation can contain any variety of dissolved minerals, the most common being quartz – in all of its crystalline forms – as well as calcite, gypsum, dolomite, barite, pyrite, and others.

Over a period of thousands of years, layers of silica cool, forming crystals of different minerals within the cavity. Different types of silica cool at varying temperatures, thus creating layers of different types of mineral crystals and/or colorful bands.

These different layers show how many different times the geode was affected by water depositing more minerals inside it. Each generation of water could have brought a new variation of mineral into the geode, combined with residues of copper, iron and other materials.

Erosion

Geodes are uncovered as individual pieces when the surrounding rock is worn away. Because a geodes' outer layer is sturdier than the covering magma or sedimentary rock, they retain their shape, but also blend into the surrounding stone. To see how they look when found, check out the videos below.

Side Trip – Quartz

Quartz is the most abundant mineral on earth (about 12% vol.), made of trigonal crystallized silica (silicon dioxide, SiO2), with a hardness of 7 on the Mohs scale.

Figure 5. Six-sided shape.

The typical shape is a six-sided prism that ends in six-sided pyramid, although these are often distorted, or so massive that only part of the shape is apparent from a mined specimen.

A quartz geode consists of a hollow rock, its core lined with a bed of crystals.

Being one of the most common minerals, quartz goes by a bewildering array of different names. The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (aggregates of crystals visible only under high magnification).

Chalcedony is a generic term for cryptocrystalline quartz. The cryptocrystalline varieties are either translucent or mostly opaque. The transparent varieties tend to be macrocrystalline.

Although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is a secondary identifier for the cryptocrystalline minerals, while color is a primary identifier for the macrocrystalline varieties. This does not always hold true, however.

Major varieties of quartz

Chalcedony	Any cryptocrystalline quartz, although generally only used for white or lightly colored material. Otherwise more specific names are used. en.wikipedia.org
Agate	Banded Chalcedony, translucent
Onyx	Agate where the bands are straight, parallel and consistent in size.
Jasper	Opaque chalcedony, impure
Aventurine	Translucent chalcedony with small inclusions (usually mica) that shimmer.
Tiger's eye	Fibrous quartz, exhibiting chatoyancy.
Rock Crystal	Clear, colorless
Amethyst	Purple, transparent
Citrine	Yellow to reddish orange, greenish yellow
Rose quartz	Pink, translucent, may display diasterism
Milk quartz, or snow quartz	White, translucent to opaque, may display diasterism
Carnelian	Reddish orange chalcedony, translucent

Quartz occurs in hydrothermal veins and pegmatites. Well-formed crystals may reach several meters in length and weigh hundreds of kilograms. Erosion of pegmatites may reveal expansive pockets of crystals, known as "cathedrals."

Quartz is a common constituent of granite, sandstone and limestone.

Terms

As with other earth events and features, the unique composition of geodes can teach us about minerals, how they interact to produce structure, and ultimately the world we live on.

Definition of Chalcedony (kāl-sěd'n-ē)

Definition of Quartz

Quartz is an essential constituent of granite, and abounds in rocks of all ages. It forms the rocks quartzite (quartz rock) and sandstone, and makes most of the sand of the seashore. Origin: G. Quarz. Source: Websters Dictionary (01 Mar 1998)

Definition of Cryptocrystalline

Definition of Crystal

Definition of Vug