The purity seal is a somewhat distinct mark in the Imperium of the 40k universe. They are awarded by Space Marine Chaplains to brother marines as blessings. I also see them as awards given to those worthy enough; those people who perform great feats, or serve the Emperor moreso than your average citizen. These are easy and fun to make.
There are many different ways to sculpt these seals. As you can see, I've used simple modelling clay for one, and Super Sculpy for two others. The first thing I do is to make a ball of material about 1" or so in diameter. On a smooth surface; I find the lids to juice concentrate containers very useful; center the ball and flatten it out. The metal lids I just mentioned are nice to use for several reasons. They are just the right size of a typical seal, they are flat, and since they are metal, you can bake the Sculpy right on them.
Once you have your pancake, the rest is pretty straightfoward. Scoop out the center of the pancake and build a ridge along the edge. Now, just choose the design of your seal and go for it. The first one I did was a typical skull purity seal. The skull represents the Emperor and his great deeds, and that, even in near-death, he protects. The second seal is an Inquisitorial seal. I sculpted the =][= design, and was left with a pair of blank portions on the seal. I sculpted a small skull to the left of the =][=, and decided on a maltese cross on the right. The third seal is a simple maltese cross. I figure it can stand for something along the lines of Terminator Honours, or proof of a great warrior.
The plan now is to make a mold of each of these, and to cast them in plastic resin.
Below, you can see the painted seals. I used a flat black primer coat follwed by a gloss red, to lend a waxen feel to them. To see these on a finished costume, check out my Inquisitor costumes.