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The Earth's crust is made of rock, which is composed of minerals. Minerals are solid, naturally occurring non-living materials that have a structure of crystals. There are hundreds of different kinds of minerals, but about 90% of the ones on Earth are silicates. Silicate minerals include oxygen and silicon along with some other combination of elements like iron, aluminum or potassium. Since each mineral is made of different elements, each has a unique set of properties. For example, diamonds are very hard, iron is strong and halite (salt) tastes good on food.
Which of the following is not true about minerals?
A They always include oxygen and silicon.
B They are naturally occurring materials.
C They each have unique properties.
D They have a crystalline structure.