Sailor Crystals are more advanced forms of starseeds. They come from planetary crystals. Sailor crystals represent a primal force in the universe that is at once both ancient and modern.
Sailor Venus' sailor crystal as shown in the manga.
Formation of Sailor Crystals
While each starseed usually gravitates to a nascent life form, there are times when starseeds are attracted not to biological lifeforms, but the energy signatures of forming worlds. These seeds will rush by the billions into the cores of these forming planets, their energies combining under the gravitational pressure, merging and creating much larger, more powerful and complex crystals that form the hearts of these worlds. Intimately tied to their structures and unique energies like normal starseeds, they exist in a dispersed form, suffusing their planetary body.
Becoming a Sailor Crystal
When a compatible host body is conceived, which is very rare because their bodies would have to be almost totally in tune with the vibrations of their planet, the energy of the planetary crystal will gravitate to it and infuse it with planetary power. These individuals will then possess not starseeds, but sailor crystals. There can be more than one such person per planet because the energy of the planetary crystal cannot usually be safely contained by just one body. There is generally plenty left over.
The Power of Sailor Crystals
Sailor Crystals are more powerful than starseeds because they possess the capacity to resonate with and channel the energy of their host planet. They are also capable of not only perfectly recording the memories and experiences of their hosts, but their physical structure, as well. They give a body far greater strength, speed, endurance, and longevity, than otherwise conferred by a starseed, as well as enhanced traits in keeping with the attributes of the world that gave rise to them.
Interaction with Hosts
Their energies can either be fully manifested at all times, resulting in a being who is literally the living avatar of their world, or they can be partially or wholly suppressed, resulting in a being who superficially seems to be just like any other starseed-carrying individual. In the later case, there is a danger that when the sailor crystal is fully activated, the latent patterns of a past host body might overwhelm that of the new host, but this rarely occurs. In short, holders of a sailor crystal are virtually immortal, so long as their crystals are not destroyed. Their bodies may die, but upon the generation of a new suitable body, they are reborn just as before. Thus holders of sailor crystals who usually hold the potential to be the strongest beings on their own worlds have often been feared, revered, and even worshiped as gods.
This differentiation between sailor crystals and starseeds is not made in the anime, but is in the manga, where Sailor Galaxia regards starseeds as trash, but not sailor crystals, which she hunts ruthlessly.