The Quartz

The Quartz is the most common mineral, because it's made by two chemical elements: silicon and oxygen, which are abundant on Earth.

Quartz has many of different varieties which differ from each other from the size of their crystals.

all this varieties can be divided in three categories based on the size of their crystals:

·        Large single crystals.

·        Microcrystalline aggregates of small crystals.

·        Cryptocrystalline aggregates of very small crystals (invisible to the eye).

Even though share the same chemical structures and composition, each one of those categories has gems that looks very different from each other.

Single crystal.

Amethyst

Amethyst: has always been one of the most-prized gems, either for its vibrant color and for its affordable price, way much cheaper than rubies and sapphires.

The hues vary from a violetish purple to a reddish purple, which is the finest quality.

Usually, the amethysts that we see on the market are free of visible inclusions and even colored, even though sometimes is possible see a color zoning. the rough crystal is often color zoned, with the color concentrated on the tips. Amethyst are heat treated very often to improve their color, and also removes some brown hint that makes the stone less marketable.

The main sources of Amethyst are Brazil and Africa; in particular, Zambia produces the best qualities gems.

Brazilian's gems are usually medium-quality and usually are suitable for heat treatment that turn its color in a citrine's Yellow.

Citrine Quartz

Citrine: is very rare in nature and often is confused with topaz! Its beautiful yellow-orange color is caused from a trace of Iron in its chemical composition. The most prized Citrine is free of inclusion and color zoning. the best color is a saturated yellow to reddish orange.

Most citrine that we can found on jewelers are heat treated Amethyst.  

Ametrine Quartz

Ametrine: this gem contains both amethyst and Citrine colors. Cutters usually prefer polish the stone with an half-half effect that shows both colors. Bolivia is only one source of natural Ametrine in the World.

Some Ametrine we see on the market are the result of heat treatment and Irradiation of color-zoned Amethyst.

Smoky Quartz

Smoky Quartz: it used to be worn as mourning gem in late Victorian period. This quartz displays an orange-red pleochroic flashes. Because of its color very often is confused with topaz, but it’s much cheaper!

sometimes is heat treated to lighten the color and get a yellow hint similar to citrine.

Rose Quartz

Rose Quartz: of this variety the color range goes from light to dark pink. When the crystal is found transparent and inclusion free, is usually faceted and used in jewels.

When contains small inclusions it can sometimes shows asterism with a six-pointed star.

the best quality usually comes from Brazil.

Rock Crystal

Rutilate Quartz

Rock Crystal: is the colorless transparent quartz, usually inclusion free. This variety is abundant and usually is found in big size. in the past has been used very often as a Diamond simulant.Sometimes has some eye-visible inclusions, that don’t affect to the gem’s value, actually contributes to increase its value. Is this case the gem is called Sagenitic Quartz and in the trade get different names according to the different type of inclusion you can fine inside. For example,  a very popular one is the Rutilate Quartz: rutile’s crystals looks like a needle-like inclusion and inside the quartz creates this fascinating designs. Rock crystal are often heated and Dyed to get a color similar to stones more expensive, this treatment is called Quench-Crack.

Microcrystalline aggregates.

Gems that belongs to this variety are made by more small crystals connected to each other that, with some needle-like inclusions, usually interact with light producing some fascinating optical effects.

Tiger’s eye: the surface of this semi-translucent stone has a pattern of yellow and brown bands that reverse the color if the stone is turned. Actually, of this stone, we can have different varieties, like grayish blue (hawk’s eye). The phenomena of Chatoyancy that characterizes those gems is caused by their fibrous structures oriented parallel.

Aventurine quartz: in this gem the aventurescence is caused by some small mica inclusions that reflects the light.

Cryptocrystalline aggregates

The chalcedony is a cryptocrystalline quartz, it means that is an aggregate of crystals so tiny that are invisible to naked-eye. The rough chalcedony has a very unusual structure, called botryoidal, that looks like a bunch of grapes. Because is made from the mix of a lot of tiny different crystals, the final gem can display different bodycolors and different pattern from translucent to opaque.

Carnelian

Chrysoprase

As translucent gems there are some in just one uniform color like the intense green of Chrysoprase, the vibrant blue of the Chrysocolla-in-Chalcedony and the warm brownish color of Carnelian and Sard.

Crysocolla-in-Chalcedony

Aggiungi didascalia

There are two varieties of Chalcedony that display parallel stripes of contrasting colors: Onyx, with black and white bands, and Sardonyx, with brown and white bands. This color banding make this gems suitable for cameos and other carving designs.

Agate in Geode

Agate is also a chalcedony with banded structures which can vary in color and transparency, and the bands are not parallel, actually they are curved and irregular. Collectors love this stone because of the magnificent pattern that it displays; sometimes the stripes create scenery that resemble real landscape! 

Agate is found in deposit call Geode. 

Fire Agate, it displays Iridescence

Dendritic Agate

Moss Agate

All this differences produces a big variety of Agates like: Moss Agate, Dendritic Agate and fire agate that displays iridescence.                                   

Leopard Jasper

The completely opaque chalcedony, doesn’t matter which color, is called Jasper. Sometimes the Jasper displays on its surface a pattern that resemble the coat of the leopard, this stone is called on the trade Leopard Jasper. 

Bloodstone

The Bloodstone is a special variety of Jasper. It has a dark green bodycolor and is characterized by some dark red spots that resemble drops of blood.  

Phenomenal gems

Phenomenal gems

There are some stones that interact with light in ways that produce unusual optical effects (phenomena). Some of those phenomena belongs to specific gems, and sometime other gems may, or may not, show some other effects, depending of their quality and variety.

Phenomena are divided in 8 groups:

chrysoberyl

1. CHATOYANCY; consist of bands of light  reflecting from needle-like inclusion, parallel. If cutted properly, the stone will reflect the light creating a shiny line that looks like a cat’s eye. The best type of cut to enhance the phenomena is a “cabochon” that concentrates the eye across the dome of the cabochon. The top quality gems shows an eye sharp and intense. The most valuated chatoyant gem is the cat’s eye chrysoberyl that, in addition to chatoyancy, show another optical effect called “milk and honey”: it’s a 2 tone effect  seen when a chatoyant  gem is at right angles to a light source. One side turns milky and the other a dark honey color. In an other gem, a “tiger eye quartz”, the phenomena is shown even without a direct light, or a faceted cut.

star sapphire

2. ASTERISM; the effect results from crossed chatoyant bands. 
The light reflects from tiny needle-like inclusions oriented in different directions. Usually, if a stone shows this kind of inclusions, the cutter prefer the cabochon shape to enhance the phenomena, otherwise the gem will look just translucent. Actually, if the stone shows a sharp nice star, it will increase its value. Star sapphire and star rubies are the most valued gems that displays asterism.

       

3.   AVENTURESCENCE; is a glitter effect   made by some flat inclusions inside the stones. In natural gems this phenomena is typical of  a variety of quartz called “Aventurine”, but is very common  in low cost goldstones, that are just glass or plastic with filled with glitter.

opal

 4. PLAY-OF-COLOR; the Opal shows this phenomena. Opal is not a Chrystal,  is made of microscopic spheres that breaks up the light into spectral colors depending of the size of the      spheres and from the way they interact with light, the stone will show different body color and different pattern of phenomena. Usually in the trade is preferred a dark body color where the effect is more visible. The most valuated is the “fire opal”, red gem that may or may not shows play-of-color.

moonstone

5. ADULARESCENCE; phenomena typical of the moonstone. The effect is shown like a light blue clouds that appears when the light hits the gem. It’s hard to see the phenomena in a rough stone, the cutter usually polish the stone in a cabochon to enhance the effect.

Labradorite

6. LABRADORESCENCE; is the Labradorite's  phenomena; is a flash of color that appear when light strikes the stone from specific angles. The cutter always try to cut the stone while the effect is more visible from the front.

mother-of-pearl

 7. IRIDESCENCE; in nature we call iridescence the rainbow color that we see when a light bean is broke in a spectral colors. On jewels we can see this effect on different stones, for example Ammonite, fire Agate and also on Pearls and Mother-of-Pearls where, in this case, the phenomena is called ORIENT.

fire agate

alexandrite chrysoberyl

8. COLOR CHANGE; it’s a very rare phenomena that shows a effective change in color of a gem from daylight to incandescent light. The most well-known and valuated color change gem is Alexandrite Chrysoberyl, that in its best quality turn from vibrant green to rich red. Emerald by day and Ruby by night. Some other stones can show this effect: Sapphire, Garnet, Spinel.