Celestite

Celestite (strontium sulfate) has a sky (or celestial) blue color that is unique in the mineral kingdom. The name stems from the Latin word Caelertis, meaning "of the sky." Popular lore says that celestite came from the star group called the Pleiades, and that it holds the wisdom of the ages.

This ethereal mineral is believed to aid personal creative expression, help reduce stress, and provide peace of mind. It is also said to help one adjust to higher states of awareness, provide access to and transfer of information from the angelic realms, and assist in clairaudient endeavors.

First found in Italy in the 18th century, celestite crystals have since been unearthed in New York, Michigan, Ohio, Colorado, California, Poland, Czechoslovakia, Germany, and Mexico. A nodular form, like a geode with an exterior of sedimentary rock and a cavity full of brilliant crystals, is found in India. The finest specimens of any type are from Madagascar.

Celestite has the same structure as barite (BaSO4), and forms very similar crystals. While barite is not typically blue, occasional blue specimens are usually misidentified as celestite. The two may seem identical by ordinary methods, but a flame test can distinguish them. Scrape some dust from the crystals into a flame -- if the flame turns a pale green, it is barite; but if the flame is red, it is celestite.

The red shown in the flame test is due to the presence of strontium, an alkaline earth metal. Strontium compounds are important components in signal flares, tracer ammunition, and fireworks (because of the red flame), color tv picture tubes and computer monitors (it absorbs x-rays and improves the brilliance of glass), and in making permanent ceramic magnets and refining zinc and beet sugar. Due to its extreme reactivity with air (it will ignite spontaneously when finely powdered), strontium is always found in combination with other elements in minerals like celestite and strontianite, and celestite is its primary source.

Celestite (a.k.a. Celestine): SrSO4, strontium sulfate
Color: usually blue, but may also be colorless or yellow and contain tints of red, green, and brown
Habit: generally well-developed tabular or bladed crystals; also nodular, fibrous, or granular
Hardness: 3 - 3.5
Luster: vitreous
Transparency: crystals are transparent to translucent
Cleavage: perfect in one direction, less so in another direction
Fracture: conchoidal
Specific gravity: 3.9+ (above average for translucent minerals)
Streak: white
Other: red color in flame test, some specimens fluoresce under UV light, may fade if exposed to direct sunlight

About Pearls

Throughout much of recorded history, a natural pearl necklace made up of matching spheres was a treasure of almost incomparable value -- in fact, the most expensive jewelry in the world! Before the creation of cultured pearls in the early 20th century, pearls of any sort were so rare that they were reserved for the noble and very rich. To prove the great wealth of Egypt, it is said that Cleopatra won a contest with Marc Antony to give the most expensive dinner in history by crushing and adding to a glass of wine a single large pearl worth about 30 million sesterces (or about $4,700,000)!

However, around 1907, several Japanese men working simultaneously discovered that they could entice oysters to create pearls. As the process has been refined, these cultured pearls have become virtually indistinguishable from natural pearls, making the most valuable gem in the world become the beauty that we can use in our beading designs today for just pennies apiece!

Natural pearls are formed when an irritant or parasite enters an oyster or mussel and cannot be expelled. To reduce irritation, the mollusk uses its mantle to coat the intruder with the same secretion it uses to build its own shell, nacre (also called mother-of-pearl). Continuous layers of nacre grow like an onion skin around the irritant, until the pearl is harvested or it becomes too large for its host. Pearls vary in shape, depending on the shape of the foreign body being coated.

However, mollusks are normally very good at expelling these foreign bodies before the pearl-forming process begins -- so naturally-occurring pearls are found in perhaps one of every 10,000 bivalves. On the other hand, with culturing techniques, every one of those 10,000 creatures could create a pearl -- or more than one! Culturing is a process that is simple in concept but requires great skill to perform. In essence, an irritant (commonly a piece of mother-of-pearl from a Mississippi river mussel) and a small piece of mantle tissue (a "graft") are inserted by a specialized technician. Great care must be taken, or the oyster or mussel may be damaged in the process or even die.

After a short rest, the mollusk begins to coat the nacre "seed" just as it would a natural irritant. Layer upon layer grow, until the pearl is large enough to be harvested. Cultured pearls are composed entirely of nacre, just as natural pearls are, so they exhibit the same structure, color, and gorgeous luster as their more expensive counterparts. Additionally, a wide variety of seed shapes may be used, to create interesting new pearl shapes, from the now-familiar rice and potato pearls to the more unusual baroque, matchstick, drop, and cross shapes.

The crystalline structure of nacre (which is a combination of the mineral aragonite and the protein conchiolin) reflects light in a unique way, giving pearls their high luster. Light rays reflect not only off the surface of the pearl, but also off the concentric inner layers of nacre, which act like tiny prisms and create iridescence within the tiny sphere. This distinctive glow fostered the belief, long ago, that pearls were moonbeams which fell into the ocean and were eaten by oysters. Consequently, they have been believed magical, and symbolized the moon, purity, spirituality, and virtue for centuries.

Caring for your pearls: Avoid contact with hair spray and perfume. Wipe with a soft, damp cloth and store them in a soft cloth. Wear them often -- your natural body oils help to keep them from drying out.

Check out our large selection of freshwater pearls and mother-of-pearl shells.

How to Choose the Right Rock Tumbler

Rock tumblers make very popular holiday gifts for adults and kids alike, but it's often difficult to decide which one you should buy. First, you should consider three basic properties:

Rock Hardness. The stones you tumble together must be of the same hardness. For example, you would want to tumble quartz with other quartzes, such as jasper or agate. If you tumble softer rocks with harder ones, the softer rocks will end up in little bits and pieces. In general, rocks with a hardness of less than 5 Mohs are more likely to crumble than polish if tumbled.

Barrel Size. The barrel must be at least 2/3 full, and may be no more than 3/4 full. If you think you'll want to tumble fairly large pieces of rock, you may want to consider one of the larger-barreled models. On the other hand, if you feel that you won't often be able to fill a larger barrel, you might want to look into a smaller one.

Number of Barrels. You may wish to consider a multiple-barrel tumbler. Some of the advantages of more than one barrel are 1) not having to wait for the rocks you found last week to finish tumbling (five weeks from now) before you start tumbling the rocks you found this week; 2) being able to tumble rocks of different hardnesses at the same time; and 3) having a barrel for each member of the family!

Which kind of tumbler is right for you? Each of the two basic forms of rock polisher -- rotary and vibratory -- has its advantages.

Rotary tumblers are an excellent way to get started tumble-polishing rocks. Good-quality tumblers, such as the Lortone models we carry, are much cheaper than comparable vibratory units. Getting good results on rocks that are Mohs 5 or greater in hardness is not difficult, and generally your batch will require checking only once every day or two. You can also tumble-polish metals in these units. Rotary tumblers with rubber barrels -- like the Lortone models -- are also noticeably quieter than vibratory tumblers.

Rotary tumblers are best for producing seaglass (while both rotary and vibratory tumblers can be used to achieve a high sheen on tumbled glass).

On the other hand, they take 4 to 6 weeks to produce results. This can be a sizeable disadvantage for some people.

Vibratory tumblers such as the Gy-Roc Model B can finish a batch of rocks in little more than a week. Types of appropriate loads are more varied, including very soft organic materials like ivory, with a hardness level down to Mohs 2-1/2. One sort of processing that is possible only with vibratory units is dry polishing.

However, this versatility brings with it a fair degree of complication. Vibratory tumblers are also quite noisy, though this can be moderated to some degree by placing them on a piece of carpeting or other padding.

For many people, the initial high cost of these units is decisive. If, however, speed is important to you, you can process much more rock in the same amount of time. Capacity can be increased by piggy-backing additional inexpensive hoppers onto the basic unit -- up to three total on the Gy-Roc.

So which sort is better for you? It depends entirely on your needs and resources.

View our selection of Lortone Rock Tumblers and Gy-Roc Vibratory Tumblers.

Geology Word Puzzler

What is causing the ongoing eruption of Mount St. Helens in Washington state?

A convergent plate boundary (subduction zone). Convergent plate boundaries come in several flavors, but they share one thing in common -- plate collisions!

In a contest between a dense oceanic plate and a less dense, buoyant continental plate, guess which one will sink? The dense, leading edge of the oceanic plate actually pulls the rest of the plate into the flowing asthenosphere, and a subduction zone is born! Where the two plates intersect, a deep trench forms.

Geologists aren't sure how deep the oceanic plate sinks before it completely melts, but we do know that it remains solid far beyond depths of 100 km beneath the Earth's surface.

When the subducting oceanic plate sinks deeper than 100 kilometers, huge temperature and pressure increases make the plate "sweat." Well, not exactly, but the uncomfortable conditions force minerals in the subducting plate to release trapped water and other gasses. The gaseous sweat works its way upward, causing a chain of chemical reactions that melt the mantle above the subducting plate.

This hot, freshly melted liquid rock (magma) makes its way toward the surface. Most of the molten rock cools and solidifies in huge sponge-like magma chambers far below the Earth's surface. Large intrusive rock bodies that form the backbones of great mountain ranges such as the Sierra Nevada form by this process.

Some molten rock may break through the Earth's surface, instantly releasing the huge pressure built up in the gas-rich magma chambers below. Gasses, lava and ash explode out from the breached surface. Over time, layer upon layer of erupting lava and ash build volcanic mountain ranges above the simmering cauldrons below.

An example of this kind of convergence produces the spectacular volcanic landscape of the Northwest. Off the coast of Oregon, Washington, Alaska and Canada a small plate, the Juan de Fuca, dives beneath North America. This type of convergent plate boundary, called a subduction zone, is known for producing historic earthquakes of great magnitudes (and the current eruption of Mount St. Helens).

~from U.S. Geological Survey.

We'd love to hear from you! Have questions or comments about the website, or just want to tell us about your latest rockhounding adventure? Email us at we_rock@mamasminerals.com!

Read about rose quartz, geodes, and identification of rocks and minerals in the Fall 2006 issue of RockZ NewZ

Read about rose quartz, geodes, and identification of rocks and minerals in the Summer 2006 issue of RockZ NewZ

Read about fluorite, fluorescent minerals, and cleaning quartz in the Summer 2005 issue of RockZ NewZ.

Read about vanadinite, building fountains, and identifying meteorites in the Winter 2004 issue of RockZ NewZ.

Read about malachite, tumbling grit, and how to pan for gold in the Summer 2004 issue of RockZ NewZ.

Read about amethyst, fossil preparation, and field tools for the rockhound in the Spring 2004 issue of RockZ NewZ.

Read about galena, petrified wood, and collecting micrometeorites in the Winter 2003 issue of RockZ NewZ.

Read about iron pyrite, tumbling seaglass, and stony meteorites in the Summer 2003 issue of RockZ NewZ.

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