An unknown mineral specimen is not scratched by a penny and does not exhibit cleavage. the mineral is most likely to be

Quartz

Explanation:

Quartz is a resistant mineral whose hardness is 7, according to the Mohr's hardness scale. It is made up of silicon and oxygen and very abundant on the earth's continental crust, after feldspar. It is a very hard mineral that crystallizes in the hexagonal system.

It cannot be scratched by a copper penny and also cleavage is absent. They are the most stable rocks that resist weathering and erosion.

Thus, it is a very important mineral that forms the majority of sedimentary igneous and metamorphic type of rocks.