Type I quartz colored varieties gemstone - Color caused by color center

"Rock Crystal" is the name for clear, colorless pure quartz crystal. Quartz is the second most abundant mineral in the Earth's continental crust with an overall formula SiO₂. Rock crystal can be found in some form in any geological environment that allows the formation of quartz in general. Large, well-formed and transparent crystals can be found. For nice crystals to form, conditions in a geological environment need to change more or less gradually and slowly over a long period of time.  A single rock crystal may have grown over a period of several million years during the uplift and folding of the mountains.

There are many different varieties of quartz, common colored varieties include citrine, rose quartz, amethyst, smoky quartz, blue quartz, and others, which are semi-precious gemstones. The color formation of rock crystal’s varieties will be reviewed as following.

Amethyst has been known as the most highly prized form of quartz. The color can vary from a pale purple to a dark purple. Very often the color is unevenly distributed and is most intense at the tips of the crystals.  Amethyst owes its color to the presence of iron built into its crystal lattice. Some of this iron sits in sites normally occupied by silicon and some is interstitial. The iron impurity is usually in the +3 valence state to substitute Si.  For natural amethyst, Gamma ray radiation from nuclear decay in the surrounding rocks can make charge transfer from an iron lattice site to an interstitial iron, and form +4 valence state of iron (purple color centers). This +4 iron absorbs certain wavelengths (357 and 545 nanometers) of light causing the purple color. It is needed to have quartz that contains the right amounts of iron and then is subjected to enough natural radiation to cause the color centers to form. 

Citrine is the yellow to brownish-red variety of the Quartz.  Citrine’s yellow color is caused by quantities of Fe³⁺ impurities which form color center inside quartz’s lattice. However, Citrine is somewhat rare in nature.  Most citrines on the market have been heat treated from amethyst or smoky quartz. Natural Citrine is yellow to orange-yellow, and occurs in much lighter hues than the heat-treated material, which is dark orange-brown to reddish-brown. Virtually all heat-treated material has a reddish tint, whereas the natural specimens do not.

Inexpensive amethyst is often heated at high temperatures to produce the more profitable orange yellow citrine. Amethyst has been known has iron impurities at +4 valence state.  Iron +4 valence state related purple color centers are not as stable as iron +3 related yellow color centers at high temperature, thus at high temperature only more stable yellow color centers exist to produce yellow color.  Citrines whose colors have been produced by artificial means tend to have much more of an orange or reddish caste than those found in nature, which are usually a pale yellow.  In some Amethyst deposits, the Amethyst has been partially or fully changed over to yellow Citrine by natural means of heating. It is relatively well known that the vast majority of citrine quartz is the product of heat treating amethyst. Material from Brazil and Uruguay has often been used for this purpose. Both large and small amethyst-lined geodes are converted to citrine using simple low-temperature heat treatment in air. Citrine made by heating amethyst may be returned to a purple color by bombarding it with radiation which cause color centers transfer back to purple color centers.

Ametrine contains both amethyst and citrine in contact with one another.  The colors of amethyst and citrine are produced by iron impurities with different oxidation states within the quartz.  Purple is thought to be produced by Fe⁴⁺ related color centers, of which the Fe impurities are oxidized to Fe⁴⁺ by natural radiation emitted in nearby rocks. The golden-yellow is thought to be produced by Fe³⁺ related color centers. The different oxidation states occur due to there being a temperature gradient across the crystal during its formation and by natural radiation.  The concentration of iron is much higher in the yellow than in the purple sectors. If heating up a natural Ametrine, only the purple sectors loose their color, and the result is a crystal with 3 almost colorless and 3 yellow sectors. 

The color of smoky quartz is caused by irradiation and aluminum impurities built into its crystal lattice to form the color-centers.  In normal geological environments, the color centers formation process can only take place at temperatures below 50°C, otherwise the rate of color center destruction surpasses that of color center formation.  Thus the color of smoky quartz crystals appeared long after the crystals have grown. It is estimated that it takes several million years for a crystal to assume a deep color by natural radiation emitted in nearby rocks.

Pink quartz’s color is caused by small amounts of aluminum, Al⁽⁺³⁾, and phosphorus, P⁽⁺⁵⁾, built pairwise into the crystal lattice to replace Si⁽⁺⁴⁾, and subsequent high energy irradiation. The two electrically neutral SiO₄ groups are replaced by one AlO₄^- and one PO₄⁺ group. Pink quartz is often accompanied by phosphate minerals, like (Mn,Fe)Al[(OH)₂|PO₄]•H₂O.  Many pink quartz is translucent to transparent. Pink quartz is very sensitive to light and will pale quickly in direct sunlight. This is an indication that the color is due to color centers whose formation appears to be triggered by high energy radiation.

Colored Quartz is an affordable gemstone, and is used in various form of jewelry.  High quality colored Quartz is usually faceted for ring and pendant centerpieces, and less quality colored Quartz is most often used in gemstone beads with different shapes:  round, coin, oval, square, pillow, cube, tube, et al. With colored Quartz beads in your favorite color and shape, you even can create your own unique piece of colored Quartz jewelry, and with plenty of opportunity to customize to fit your unique style. There are plenty of low-cost colored Quartz beads available through online store and be sure to check out the discounted colored Quartz beads first to start your design.