21K gold or 21 Karat gold is 87.5% pure (21/24) and is much softer than the preceding two ratings.

What is it used for
21 karat gold is generally not publicly available. It is basically made on special requests for custom jewelry designs. Higher standards in technological performance also require 21 karat gold.

Where it is popular
21 karat gold is very popular among wealthy people or aristocrats who wish to present valuable gifts and gestures to family members, friends, or associates.

Pros
Since it is pure gold it is obviously costly

Cons
Purer gold but is softer and has a larger risk of wear and damage. 21 karat gold needs special care and handling. 21 karat jewelry when sold at times in which gold prices are high are very profitable.