Search results
Results from the WOW.Com Content Network
The Alexandrite effect has also been observed in some other minerals, such as fluorite, sapphire, kyanite, monazite, spinel, garnet, tourmaline, and rare-earth oxalates. Not to be confused with the alexandrite effect, some minerals also exhibit pleochroism. The former is a response to different wavelengths of light in general, the latter an ...
Synthetic alexandrite is an artificially grown crystalline variety of chrysoberyl, composed of beryllium aluminum oxide (BeAl 2 O 4). The name is also often used erroneously to describe synthetically-grown corundum that simulates the appearance of alexandrite , but with a different mineral composition.
Stones that show a dramatic color change and strong colors (e.g., red-to-green) are rare and sought-after, [8] but stones that show less distinct colors (e.g. yellowish green changing to brownish yellow) may also be considered "alexandrite" by gem labs such as the Gemological Institute of America. [13] [14]
Tetragonal, trigonal, and hexagonal minerals can only show two colors and are called dichroic. Orthorhombic , monoclinic , and triclinic crystals can show three and are trichroic. For example, hypersthene , which has two optical axes, can have a red, yellow, or blue appearance when oriented in three different ways in three-dimensional space. [ 5 ]
Historic recurrence is the repetition of similar events in history. [a] [b] The concept of historic recurrence has variously been applied to overall human history (e.g., to the rises and falls of empires), to repetitive patterns in the history of a given polity, and to any two specific events which bear a striking similarity. [4]
A star sapphire is a type of sapphire that exhibits a star-like phenomenon known as asterism; red stones are known as "star rubies". Star sapphires contain intersecting needle-like inclusions following the underlying crystal structure that causes the appearance of a six-rayed "star"-shaped pattern when viewed with a single overhead light source.
The date used as the end of the ancient era is arbitrary. The transition period from Classical Antiquity to the Early Middle Ages is known as Late Antiquity.Late Antiquity is a periodization used by historians to describe the transitional centuries from Classical Antiquity to the Middle Ages, in both mainland Europe and the Mediterranean world: generally from the end of the Roman Empire's ...
Hydroquinone-induced exogenous ochronosis is an avoidable dermatosis that is exceedingly difficult to treat. However, some studies show that treatment may be possible with a Q-switched alexandrite (755 nm) laser. [13] Individuals with this disorder are recommended to stop using hydroquinone-containing compounds. [2]