Three examples of mineraloids are amber, jet and lapis lazuli, Gemstones Advisor states. A mineraloid is a naturally occurring, amorphous substance that lacks the crystalline structure necessary to be classified a mineral, according to Geology.com.
Other examples of mineraloids include opal, pearl and obsidian, which are used in jewelry, according to Gemstones Advisor.
Substances must meet five requirements to be classified as minerals: they must be naturally occurring, inorganic and solid and have a crystalline structure and definite chemical composition, Geology.com states. Many substances that lack crystalline structure and sometimes chemical composition are therefore classified as mineraloids instead. Mineraloids neither cleave nor form crystals because of their structural differences. They often form in low-temperature, low-pressure environments.