Two-tone gemstones: wonders of nature

What is a two-tone gemstone?

As its name suggests, a two-tone fine stone presents two distinct colors within the same stone, which is called “polychromeThis result is due to a natural variation in the chemical composition of the stone, which has undergone changes in its crystalline structure or in its trace element content during its growth, thus creating areas of different colors. 

Main two-tone stones in jewelry: 

Ametrine:

Ametrine is probably the most famous two-tone stone. It combines the colors of amethyst (purple) and citrine (yellow) in a single gem. These two gems also come from the same family: quartz. Their final color thus determines their name, depending on the variation in iron concentration, caused by differences in ttemperature during the growth of the stone. In short, ametrine is formed when there is a temperature difference between two geological layers, which partially “discolors” the initial amethyst deposit. Indeed, a Amethyst heated to over 500°C turns yellow and becomes citrine.

“Watermelon” tourmaline:

Tourmaline is a allochromatic stone available in several colors; moreover it takes its name from the Sinhalese “thuramali” meaning “mixed-color stone”". According to the belief of the ancient Egyptians, tourmaline fell to the earth through a rainbow, which would explain its diversity of colors. The two-tone tourmaline called "watermelon" Or "watermelon" in French is recognizable by its pink heart surrounded by a green border, reminiscent of the appearance of a slice of watermelon, hence its name. Like ametrine, this combination of natural colors results from changes in the chemical composition during the growth of the crystal. Tourmalines can also occur in other color variations, such as red and blue or yellow and green; watermelon tourmaline remains the most well-known and sought-after.

Fluorite:

Known for its diversity of colors, fluorite can also appear in bicolor form. This stone often has bands of different colors, ranging from purple to green, with sometimes some touches of blue or yellow. Fluorite is also appreciated by gemologists for its clarity and fluorescence under ultraviolet light, but is still little used in jewelry today due to its low hardness, which makes it more difficult to set on jewelry.

In summary, two-tone stones are still little used in jewelry at present, because they are little known to the general public, but their unusual appearance is a real gift, making them unique and spectacular!

Other two-tone stones (not fine)

Unlike fine stones, many ornamental stones have several colours, such as the chalcedony family: carnelian, jasper or even agate which have stratifications, but also other stones, for the best known: the turquoise, malachite And jade. However, we usually speak more of colored layers rather than two-tone stones.

Which are not two-tone stones?

Some gemstones may appear to be two-tone, but this is not quite the case. This similarity may be due to natural inclusions such as oxidation or colored areas which can be found in the sapphire, revealing shades of blue, green or yellow, which are not pronounced enough to speak of two-tone.

Two-tone stones, “polychrome” can also be confused with other stones “pleochroic” which exhibit optical effects such as pleochroism, revealing different colors depending on the viewing angle and the light under which it is observed. Hardly visible to the naked eye, these stones must be observed with a tool called a “dichroscope”, allowing one to identify whether they are “dichroic” stones in which two colors can be observed as is the case for the emerald And tourmaline or “trichroic” in which three colors can be observed as in cordierite or tanzanite.

Note that a polychrome stone can also be subject to pleochroism, since these are phenomena due to independent characteristics.