Crystal-types are, whilst not on the whole overly common, fascinating to most researchers of Pokémon. When they were first discovered, they were a mystery wrapped in an enigma, containing dualities within them that seemed clearly impossible - for how could an element be so perfectly arranged in the physical way that its greatest influence would be within the spiritual range? However, as time went by, we became able to answer at least a few of the questions on this fascinating type.
Crystals form a perfect balance - coherent purity of form to the point in which essence is even more greatly affected. The practical implication of which means that, even though their physical nature is perfectly arranged, they usually draw their strength in attack and defense from what became known as the special stats (Although physical Crystal-type moves do exist). Given their unique structure, Crystal-types are resistant to most mind and chi based attacks - even to the ancient, mystical spirit energies of Dragons. Ghost-types seem to be an exception for reasons unknown - it is assumed that their negative chi is capable of being channeled through them - not with ease, nor with difficulty.
What can be said about the nature of Crystal-type attacks, that cannot be said of crystals themselves? They can bend light, creating illusions, yet they can be like prisms - shattering the illusion of white light and showing the colors within. A duality of clarity and refraction - which seems to have a devastating effect on those who manipulate shadows, or seek to cloud the minds of others. It seems, however, to be much less effective when dealing with the less ambiguous and more earthy elements, such as Poison, Ground and Grass.
A diamond may cut through steel, yet hit a diamond with a steel hammer and it shall shatter into fragments. Another interesting duality which characterizes the Crystal-types' relationship with Steel-types. Also, Crystals are still mineral - and so, they are as equally vulnerable to Grass-types as common rocks are.
Elemental strengths and weaknesses