crystallinity of seed crystals
In my field, protein crystallography, we often use seeding to improve our crystallization experiments. Typically seeding speeds up crystallization by introducing well defined nucleation cores for crystal formation. 

It can often be observed that seeding influences the crystal formation, as crystals grown from seeded crystallization conditions often have better diffraction. This is very important as all our crystals must be formed in their native solvent enviroment (water, lipids/detergents, ions etc). 

Sometimes the act of seeding can influence the crystallographic symmetry, it is not uncommon for seeding to cause crystallographic symmetry to turn into non crystallographic symmetry. Personally I have had P12(1)1 crystals generate P1 crystals due to small changes in the asymmetric unit. However my case were also rather extreme as seeding cut my crystallization time from ~3-6 months to 3-6 days.

Also note that crystal seeds, does not necessarily have to be crystalline in nature. Small glass fragments, dust particles and hairs, can also induce crystal growth (I have personally seen examples of all three on a project I have worked on). This probably occurs when the protein being crystallized is highly stable, but cannot form strong inter-molecular interactions in the crystal packing. Then having a foreign surface to interact with may help keep the protein molecules close together and allow weaker indirect interactions (such as hydrogen bonds via water molecules) to form and stabilize the crystal structure.

So in my field, seeding can have an enormous impact on crystallization experiments.