First just so you know, most molds for polymer clay are made from polymer clay (though the clay will have to be removed before hardening).
Molds made from clay won't be able to do real undercuts or 3-D (though 2-sided molds can be made for 3-D items that don't have additional undercuts--the extra flange is removed later).
Polymer clay can actually capture a great deal of detail when used to make molds, but 2-part silicone putties can capture even more and they can also be used for some undercuts and 3-D molds, and can have the clay cured right in them.
Other materials can be used to make molds for use with clay too, from caulking to rubber, plaster, wood, thermoplastic plastics, and many more.
And of course, there are many molds that can be purchased.
(Most all of those are discussed on the Molds
page linked to below.)
As for cameos in particular, the Molds
page at my site has a bunch of ways for creating cameos with polymer clay:http://www.glassattic.com/polymer/molds.htm
...click on Cameos
near bottom of list
Generally, clay cameos are created with a mixture of translucent + white clay which is pressed into a mold. The mold could be one you've made yourself from other little items, or from the impression of a rubberstamp pressed into a slab of clay, or from clay you've sculpted yourself then baked, or it could be a purchased mold or a purchased molds which you later "distort" or change the raw clay pull from that mold before hardening.
Then the molded clay is placed on smooth background clay of another color, and the whole thing is baked (helps to cool the clay in mold before removing, and not use a brand of polymer clay that's really soft).
If not using a mold, and also if putting more than one continuous image onto the clay background (like added leaves, vines, etc), the technique is bascially just doing an "onlay" onto raw clay, though in this case with very small and thin pieces.http://www.glassattic.com/polymer/onlay.htm
...click on Dimensional Onlay
category, then scroll to "Ropey, Textured, etc" onlays
Some other variations would be to create the cameo image with tinted liquid clay in a silicone mold...harden, then attach to the background clay. Or perhaps the mold could be filled completely and be thin at its edges, and have a smooth piece of clay put on top of the mold in contact with the liquid clay, then baked, to connect them at that point. Might well require some experimentation to make that work though.
You could also "sculpt" the whole cameo the same way as for a "bas relief" sculpt, then color either the cameo image or the background.
Or you could try to do that with two layers of raw clay of different colors and remove all the background color at the same level as the cameo as well as shaping the cameo image, etc.
There's more on making bas relief clay images on my Sculpting-Gen.
... click on Bas Relief
about 3/4 down list at top of page
There are various ways to actually physically "carve" polymer clay too.
One is similar to the 2-layer system I just described, but for that the bottom layer of clay can be baked if you want.
Or baked polymer clay can be carved into after curing with very tiny gouges like Donna Kato's 5 mini gouges (especially when it's warm...and don't use a brittle clay like "Sculpey" for best results): http://prairiecraft.com/polyclay/KT1.html
It's probably easier to carve the image as an impression
first though, then make a mold from the impression to use in reverse as the final cameo image if the image needs to be rounded and smooth and variously-dimensional.
There's more info on those kinds of techniques on my Carving
For a polymer clay beginner though, I'd definitely recommend using a mold, or some simple onlays till you get the hang of the material.Simulations
of cameos could also include rubberstamping an image onto baked clay with embossing ink (when heat set, will become dimensional).
Or just rubberstamping with pigment ink onto baked clay, or onto raw clay creating slight depression, but those last ones won't create real cameo dimensionality.
The last item you linked to was probably made with a different technique. It would be a little fiddly though possible to get the silver powder (mica powder or real-metal powder probably, in that case) only where it's wanted, but basically it would be a mold process then a "highlighting" process.
In other words, a mold with a flat background area around the dimensional image would be pressed into a thick slab of raw clay, then removed. The silver-colored powder would then be applied only onto to the upper
areas of the dimensional molded image...that's often done just with a finger and rubbing around gently, but in this case a small brush would be more precise especially around the edges of each upraised area. (The mold could have mostly easily been made from the impression of a deep-cut rubberstamp into raw clay, then baked.)
For more info on metallic powders and "highlighting," check out my Powders
page...highlighting will be discussed in various places there:http://www.glassattic.com/polymer/powders_metallicwaxes.htm
But there's probably more re highlighting on the Molds
page (link above) under Antiquing, Highlighting, Staining
There are also ways to remove extraneous powder that might get on the background.
(The powder could also be applied --usually after baking-- after mixing it into a clear medium to make a metallic "paint," or a metallic acrylic paint or ink could be used, but those would look a little different.