Opal
Hardness. 5.5-6.5 (Mohs scale)
Density: 1.9-2.2 g/cm³
Refractive index: 1.9-2.2
Fascinating color reflections and materialized magic. Opal is known primarily for its characteristic play of colors, i.e. opalescence.
The name of this stone probably comes from the Sanskrit word úpala, i.e. "precious stone". In ancient times it was extremely rare, and outside the former Roman Empire, expensive varieties of opal in Europe had been found only in the Červenice area in eastern Slovakia for a long time. This was changed with the discovery of large deposits in America and Australia.
It is considered to be an amplifier of the properties of its wearer and a protective stone for people born under the sign of Libra.
It is essentially a solidified hydrogel, which usually binds 6 to 10% of water in its body. Due to its amorphous structure, it is officially classified as a mineraloid.
It owes its attractive rainbow reflections to sub-microscopic spheres of silica regularly distributed within the mineral and the diffraction of light falling on them. However, there are also opals with almost no opalescense or completely lacking this quality.
Currently, the vast majority of opals are mined in Australia, but Ethiopian, Brazilian, Mexican or those from Nevada, USA cannot be overlooked. Furthermore, the presence of opals has been proven on Mars, which should be one of the proofs that water was also present here in the past in a liquid state.
Opals are not very durable, so they are not often used in engagement or other frequently worn rings. They are prone to scratches and cracks from impacts, and their porosity can cause loss of opalescence when frequently exposed to liquids; opals can lose or change color under high temperatures. Ultrasonic cleaning can also cause damage to their luster and play of color.
White opal – as its name suggests, it is most often a milky white, crystal clear or partially translucent opal. The play of colors is depicted against a white or light gray background.
It is widely found in the aforementioned Australia. It is quite popular for jewelry use due to its subtle rainbow effect.
A translucent to opaque stone with a red, orange or yellow body color and a glassy luster is called fire opal. The occurrence of a play of colours is not the rule here - even without it, it is valued for its rich hue.
It is sometimes called "Mexican opal". However, it is not only found in Mexico, but also in the USA, Australia and Ethiopia.
The rarest and most valuable type of this stone is probably black opal. Its translucence can range partially translucent to opaque with a dark to black base color, against which the play of colors in its entire range stands out.
The most famous deposit is Lightning Ridge in Australia.
Blue opal, also called Peruvian or Andean opal, is another of the beautiful varieties of this mineralite. Most of the production of blue opal, which generally reaches jewelry quality less often than other types, originates in Peru. However, it is also found in Slovakia and Nevada, among others.
Boulder opal is not a type of opal based on its color, but rather on its overall effect. In this case, the opal is grown into mostly iron-rich parent rock. Parts of the surrounding rock (matrix) are constituent to the resulting gemstone. Paradoxically, it is relatively hard as it is not a completely "pure" opal. Very often, we might encounter blue opal in boulder opal.
Dendritic opal, which has been recently gaining popularity, usually does not have any play of color. However, its white to light gray body is interspersed with branching inclusions resembling trees or ferns. These are mainly composed of manganese and iron oxide minerals.
We can also encounter the designation "Welo opal", "Wollo opal", "Wällo" and the like. It is primarily a trade name for opals originating from the Wello region in Ethiopia.
However, it has several specific properties that distinguish it, such as its higher hygrophany, i.e. the ability to absorb water. The color of its body is white to honey, translucent to milky. It has a very distinctive play of colors.
Similarly, we could also mention Slovak opals, which often takes on white and blue shades. This category can, yet again, appear to be milky, crystal transparent, or it can be found in the for of the so-called boulder opal.