The original form of a gold investment is owning physical gold. 

Physical gold can be characterized by criteria such as weight, purity and form. 

In the Western world the weight of precious metals and physical gold’s weight is traditionally regularly measured in troy ounces. 

1 troy ounce equals about 31.1 grams.

The purity of physical gold can be measured in Karat or per mill of gold contained. 

For instance, 999 gold stands for gold, which contains 99.9% of gold. 

24-Karat gold has a purity of 99.9% and is called fine gold. 

18-Karat gold has 18 parts of gold, while 6 parts (of the in total 24 parts) are a material other than gold.

Physical gold that meets certain criteria, e.g., in terms of purity can be called investment gold. 

In the European Union investment gold in the form of gold bars require a purity of at least 995 or 99.5%.