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 they conduct is non linearly dependent on the voltage level. The simplest semiconductor component is the diode, commonly understood to only pass current in one direction across it's two terminals. It actually does two things:

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

The forward threshold voltage 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.

The theory of how electrons move through crystal lattices explaining why this happens was only understood around 30 years after they entered common use.

Apparently people that lived too close to transmitters would have problems where the point contacts would "burn out" in their crystal radios. I wonder about this, as the voltages I'm using are indeed quite a lot higher than the minuscule energy contained within radio transmissions, and I do occasionally have to swap out crystals that seem to stop responding, but they do seem to recover.

List of minerals tested

I'm interested in hearing how these minerals vary across the world.

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 extracted from a handful of regions 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, approximately 270, in the case of Cornish mineralisation. Material from different locations (sometimes only metres apart) have different patterns of crystallisation due to slightly different temperatures or impurities. These changes can also affect their semiconductor behaviour. Read more on finding your own semiconductors.