A semimetal or a metalloid is a chemical element. Metalloids are elements that have the propeties of both metal and non-metal elements. It might be shiny, but brittle. It might be dull, but conducts electricity.
An approximate way to classify metalloids is to look at the Periodic Table of elements. See the double diagonal line: top left Boron and Aluminium, bottom right Polonium and Astatine. Those diagonals are the metalloids. Some metalloids are not semimetals, and some semimetals are not metalloids.
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Antimony, a semimetal, can be found naturally in the earth.
Arsenic is another semimetal that is very toxic.
Tellurium in a bottle. Tellurium sometimes is found naturally.
Germanium in a tube. Germanium can be used to make semiconductors. Silicon is used more.
Silicon crystal. Silicon is used to make semiconductors.
Boron in a tube. Boron is very hard.