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Dialga represents time and time like steel is fixed and stable much like the concept of time, steel is also man made, much like time is only perceived and is a mere concept made by mankind.
On the other hand palkia represents space and space like water is fluid and changes shape to its surroundings much like how if you move a rock in a vacuum 'space' will take its previous position, much like water would.
And giratina represents dark matter, ghosts like dark matter are unknown and mysterious, they are also theories based on a mere concept.
And how does this link into diamond, pearl and platinum? Well pearls are made inside an aquatic creature linking to water, they are also made from tiny rough objects into a smooth spherical object, demonstrating the manipulation of space.
Diamonds are made over many years from carbon material like coal to a precious gem demonstrating time, I can't think how this links to steel though.
Finally platinum is an element, we know very little about how the elements came into being and that links into ghost and dark matter.
That's my theory anyway, it's likely scientificly incorrect but I am a bit of a retard, afterall I do post on /vp/.