Semiconductors have become critical to the world in which we live, and yet the knowledge about what they are and what they do is quite limited, even with people trained in electronics. They are materials where the electrical current conducted is non linearly dependent on the voltage. In the case of a diode (the simplest semiconductor), it does two things:

it won't start conducting until a certain voltage has been reached across it's terminals
this threshold is different depending on which way round it's been connected (it has asymmetry)

The common idea of a diode only passing current in one direction fits into this - the forward threshold is much (much) lower than the reverse. With silicon diodes this is about 0.7 volts one way and 100 volts in the other. Different semiconducting materials are used to obtain different results, germanium diodes have a lower forward threshold of 0.3 volts.

Natural vs factory grown semiconductors

The silicon used in our technology may be grown in a factory, but the raw material still has to be mined. Mostly this comes from high purity quartz mined from parts of China. This material is purified further and grown into mono-crystalline blobs of silicon metal.

Natural semiconductors, such as pyrite or galena grew millions of years ago

List of minerals tested