Questions about casting are starting to come up more often, as I am increasingly moving toward custom orders these days.  So let me dispel the mystery.

When you cast something, you replicate it, you make a copy so to speak, in any metal the casting service offers.  Mine offers just about every metal on the planet, including anti-tarnish (argentium) silver and bronze, and they are green certified, meaning they only use recycled metals.
When you take your model to casting, you can go two ways: one, you can have a mold made.  With a mold, the liquid silicone or rubber fully encloses your model, taking a three dimensional impression, and adding a little “canal” that the metal flows into.  Once it is hardened, the mold is sliced in half and your model is removed.  For instance, I made my little silver frame pendant from sterling wire first, which I soldered onto sterling plate.  Then I took that in to have a mold made.  A mold for a small item like that is about $15.

Silicone Ring Mold for Flower Ring
Your other option is to go “straight to casting” without a mold.  This is cheaper (about $4 for a small item not counting the metal), but it only works when your object will liquefy or burn out during the process.  That means you can use wax or organic material (or plastic, actually).  For my melted rings, I use wax and go straight to casting.  For some of the plain rings, I have also bought finished wax models online and then had them cast.  I use mostly wax sheeting, it’s easiest to work with, but I also have wax bezel wire (with a step inside that forms the seat).  But making a bezel with wax bezel wire is very hard to do, the wax softens and melts in your hand, and the bezel comes out thick and globby.  It’s much better to make a bezel in silver.  Some model makers who are very good, use hard wax and file it down to make a perfect shape (like a conic bezel for instance), or they use a special drill to create a seat for a stone that fits exactly.  I’m nowhere near good enough for either.
In organic material, you can use leaves, twigs, patterned paper and insects for instance.  But if the pieces are too thing, like leaves, you have to add wax sheeting in the back first, or lots of clear nail polish on both sides.  With my twig rings, I first had a few twigs cast in silver and then I played around with them.  The ones I liked the best I shaped into rings, soldered them shut, and then went back to casting, this time having a mold made.  Each casting costs a labor fee ($3.00 – 4.00 for my small stuff), plus the cost of the metal, which is recalculated daily based on market prices.

One thing to watch out for when you make a casting, is the 3-5% shrinkage of the final product, which can be a lot when you need an exact fit for a stone.
Also, when you get the finished product, it’s not actually finished.  You have to cut the sprue and file down the metal.  The sprue is the little piece of canal left after casting – where the silver flowed into the mold (the casting service clips it down but not all the way because they don’t want to ruin your piece).  Then you have to tumble your piece to shine it up again, or you have to polish it.  Otherwise it looks totally matte.
All in all, I love the casting process, and I can see that many etsy shops clearly use it.  All my stacking rings are castings – Imagine how many times I’d be making them by hand, it would be crazy.  My twig jewelry is cast also, and my wax rings are too although I don’t have molds for those (I just make each one from scratch). 

Fresh Castings of Wax Rings, Sterling Silver