Fool's Gold can be one of three minerals. The most common mineral mistaken for gold is pyrite.
A naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. 
An intrusive igneous rock made of plagioclase feldspar and amphibole and/or
Gneiss is a coarse-grained, foliated metamorphic rock that commonly has alternating bands of light and dark-colored minerals. 
Granite is an igneous rock that is composed of four minerals.
Sterling silver is the standard of quality for articles containing 92.5% silver and 7.5% copper (and/or other alloys).
The fineness of jewelry gold is stated as the number of parts in twenty-four that are gold. Thus, 24 karat gold is pure gold; 12 K would be an alloy that is half gold and half copper or other metals. 
White gold was originally developed to imitate platinum, and is usually an alloy containing 25% nickel and zinc. If stamped 18 karat, it would be 75% pure gold. 
A rock is an aggregate of one or more minerals; or a body of undifferentiated mineral matter.