Opal, being amorphous, is not really a mineral but a mineraloid. One of the scientifically accepted standards defining a mineral is that a mineral must have a crystal structure, which opal lacks. Many scientific groups and references, including the acclaimed Dana's System of Mineralogy, categorize Opal together with the "true" minerals. For this reason, Opal -- as well as other amorphous types that fall under the definition of a mineraloid.
Some think it is in the quartz group.
Opal has an amorphous or non-crystalline structure, meaning it lacks a repeating atomic arrangement like that found in crystals. Instead, opal is made up of tiny silica spheres arranged in a disordered pattern, which gives it its beautiful color play known as "play-of-color."
Opal is not classified as a true crystal; instead, it is a mineraloid. Unlike crystals, which have a defined and ordered atomic structure, opal has a more irregular arrangement of silica spheres. This unique structure contributes to its characteristic play-of-color, making opal a popular gemstone despite its non-crystalline nature.
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The types of Opal that i am familiar with are White Opal, Fire Opal, Boulder Opal, Black Opal, Crystal Opal, Light Opal, Doublet Opal, Triplet Opal, Common Opal, Synthetic Opal, and Immitation Opal.
Crystal's grow by a process called nucleation.
The majority of minerals are crystalline materials.Examples: opal, halite, topaz.
Opal is known for its cleavage, which is the tendency of the stone to break along certain planes due to its crystal structure. This can give opal a characteristic pattern of fractures or cleavage surfaces when it is damaged or cut.
Opal is an amorphous mineral, which means it lacks a true crystal structure. It typically exhibits a smooth, glassy texture. Opal can also display a unique play-of-color caused by the diffraction of light, giving it a vibrant and iridescent appearance.
Opal is a hydrated, amorphous form of silica, ie sand. Silicon Dioxide, SiO2. Rather than a crystal formation, opal consists of tiny, tightly packed spheres of SiO2, with 3% to 20% of the mineral's weight as molecular water trapped amongst them.
There are 5 basic types of opal. These types are Peruvian Opal, Fire Opal, Girasol Opal, Common opal and Precious Opal.