Mica is a naturally occurring mineral that comes from the ground in rough, rock like blocks. Mica is nature's most perfect insulation. It provides outstanding insulation and withstands extremely hostile environments. Arcing will not cause carbon tracking, it withstands temperatures to 1200 Deg. Fahrenheit for muscovite and 1800 Deg. Fahrenheit for phologopite. It is relatively unaffected by changes in humidity, pressure or heat.
These are the two main types of mica used for producing parts. Muscovite mica is the most commonly used of the two.
Natural mica comes out of the ground in pieces that look like rocks. They must be split open and then the rough areas must be cut away. Most of what we deal with only has a usable area of 4-6 inches (100-150mm). While larger pieces are available the cost goes up exponentially with the size.
The short answer is; it is a naturally occurring mineral that comes out of the ground. Most of the natural mica used for parts comes from India. The reason is because of the mineral veins of mica run close to the surface in India while in North America the mineral veins tend to be a hard rock deep, narrow shaft mining which makes it more economical to import.
Mica is relatively unaffected by time. It is a mineral and has no expiration date and unless damaged by impact can last in the post-manufactured form indefinitely.
Mica was the original material in many applications. One of the reasons it is not found more commonly is the design engineers are not familiar with the material because it is not considered a cutting edge material; even though it is millions of years old and is superior in many ways to more expensive, manmade materials.