Topaz

Topaz is a popular silicate gemstone known for its hardness (8 on the Mohs scale) and wide variety of colors. It is used extensively in jewelry, with the most common colors being blue and yellow/orange. 

Characteristics and Properties
Topaz is an aluminum fluorine silicate mineral with the chemical formula Al₂SiO₄(F, OH)₂. 
Hardness: Topaz scores an 8 on the Mohs scale of hardness, making it a durable option for everyday jewelry.
Cleavage: Despite its hardness, topaz has perfect basal cleavage, meaning it can split along certain planes if struck with enough force, requiring care during cutting and setting.
Color: Naturally, topaz is often colorless or white, but trace impurities or defects can result in a wide range of colors, including blue, brown, green, yellow, orange, red, pink, and purple.
Luster: It has a vitreous (glass-like) luster and takes an exceptionally fine polish.
Pleochroism: Topaz is pleochroic, which means different colors can be seen when viewing the gem from different angles. 

Varieties and Colors
Topaz varieties are often referred to by trade names based on their color, with some hues being rarer and more valuable than others. 
Imperial Topaz: This is one of the rarest and most valuable varieties, typically a golden yellow to pink-orange or reddish-orange hue. It was named in honor of the Russian royal family in the 19th century.
Blue Topaz: The most popular color on the market today, natural blue topaz is very rare and usually pale. Most blue topaz gems are the result of irradiation and heat treatment of colorless stones, resulting in shades like:
Sky Blue: A light, soft blue.
Swiss Blue: A vibrant, intense blue.
London Blue: A deep, steely blue with green undertones.
White Topaz: This is the colorless or transparent variety, often used as an inexpensive diamond substitute.
Mystic Topaz: A colorless topaz that has been artificially coated with a thin metallic oxide layer (chemical vapor deposition) to give it a multi-colored, iridescent, rainbow-like sheen. 

Origin and Use
Major sources for topaz include Brazil, Russia, Pakistan, Afghanistan, Sri Lanka, and Nigeria. In Australia, gem-quality topaz is found in New South Wales and Queensland. 
Topaz is primarily used as a popular gemstone for jewelry, including rings, pendants, and earrings. Orange topaz is a birthstone for November, and blue topaz is a birthstone for December, adding to its popularity. Industrially, topaz is used as a raw material for refractory materials due to its ability to retain strength at high temperatures.