Gemstones and Precious Stones

Gemstones are the sexy minerals. If minerals are like different sorts of people, gemstones are the supermodels. If mineralogists are like zookeepers, who collect and classify all the different animals, gemologists are like butchers, who focus on the edible ones. Where the mineralogist asks “What variety of cow is this?” the gemologist asks “Where's the beef?”

Gemstone Fanciers versus Mineral Collectors

Just as beeves and cows are different names for the same thing, many gemstones have names that differ from their proper mineral names. Olivine is an important rock-forming mineral, for example, but as a gemstone it's called peridot. To keep the two sets of names straight, use the Gemstones to Minerals tables.

There are two ways of appreciating the mineral kingdom.

The collector of minerals loves their natural crystal form, chemical variety, fluorescence, rarity the personality of minerals in themselves. If you're a mineral-collecting kind of person, you might find a place like Emeralds appealing, which sells only uncut emerald crystals.

The fancier of gemstones is in love with what makes minerals sexy: Their purity, color, size, optical effects and value in a word, their beauty. The rest of this article is for fanciers.

Of course these categories overlap. That's why I have a big Gemstones category that gives you a peek over the fence from the mineral collector's side.

Digging Gemstones

I suppose there could be a third reaction to browsing all these jewels “Where can I dig up my own?” There are gemstone mines all over the place. One place they're concentrated in is the Franklin district of North Carolina. One of Carly Wickell's favorites is the Sheffield Ruby Mine. But most mines are generally enriched the old-fashioned term is salted with extra stones. If you don't mind that, or if you're taking children with you, then these places are fine. To do better, join the rockhounds near you and follow them around:

The ultimate gemstone fanatic dreams of opening a mine. People have found valuable things in their own yards, after all. You might not have to move to Franklin. For a real-life example, read about Scott Klein's fresnoite mine deep in the California Coast Range.

Mineralogy

There are three main minerals that form carbonates:

  • Calcite (CaCO3), which comes in high magnesium and low magnesium forms.
  • Aragonite (CaCO3), which has a different structure to calcite.
  • Dolomite (CaMg (CO3) 2), a magnesium rich carbonate produced by diagenesis.

Only low magnesium calcite is stable at surface pressure and temperatures. It is therefore the most common mineral in ancient carbonates. However, most modern carbonates are composed of aragonite as this is the mineral that most biological organisms create to make their shells or skeletons. Examples of organisms that produce aragonite shells are bivalves (sea shells), gastropods (snails) and Halimeda (a green algae). Organisms that produce a calcite shell include brachipods (a rare type of sea shell) and ostrocods (a small crustacean).

Components

Carbonates can be made of several components. These are:

  • Bioclasts: Bioclasts are fragments of dead sea creatures. These include shells and corals. The creatures precipitate the carbonate in order to produce some kind of structure.
  • Ooids: Ooids are rounded grains formed by precipitation of calcite around a nucleus to produce concentric circles (Figure 3). They form in warm, shallow waters, with a strong tidal currents. Wave action may also contribute to their near-spherical shape.
  • Peloids: Peloids are sand sized grains (100 − 150 micrometers) of micro-crystalline carbonate. They are generally rounded or sub-rounded. They originate from fecal pellets, algae and mud clasts. They are sometimes found clumped together, in a formation known as a grapestone.
  • Intraclasts: Intraclasts are clast of other limestone that appear in younger limestones. They can be quite difficult to distinguish at times, as they may be made of a similar rock as that which encases it. For example, hardgrounds can from when sea water flows through carbonate sediment, lithifying it rapidly. Subsequently, the hardground may be broken up and incorporated into the surrounding sediment.
  • Micrite: Micrite is microcystalline carbonate mud, with grains less than 4 micrometers.
  • Sparite: Sparite is coarser than micrite, with a grain size of more than 4 micrometers and is crystalline. Both micrite and sparite form the matrix or cement in carbonate rocks.