Dental fillings are most commonly used to repair cavity tooth decay. There are different materials options used and are based on your individual needs. Taking into account the size and location of your cavity. Appearance is also a consideration that goes into the top of filling.

Here are some common dental filling options:

• Composite resins, or tooth-colored fillings, are a mixture of glass or quartz filler that provide good durability and resistance to fracture in small- to mid-size fillings that need to withstand moderate pressure from chewing. They can be used on either front or back teeth.
• Dental amalgam, sometimes described as “silver-colored” fillings, is made from a combination of metals that include mercury, silver, tin, and copper. Dental amalgam has been used for generations by dentists. Amalgam is very durable and more affordable than tooth-colored or gold fillings; however tooth-colored materials are more natural looking.
• Gold fillings, also called inlays or onlays, are composed of an alloy of gold, copper and other metals. Gold has been used in dentistry for more than 1,000 years due to its durability; however, gold is more costly than amalgam and not natural looking like tooth-colored fillings.

The ADA has more info here