Have you ever needed a gemstone of a specific shape, color or size, but can't find them at any stores (or online)? As cosplayers, we often encounter this problem, which is why casting our own gems is often the only choice we have.
There a few different ways to cast gemstones (and materials that you can use) but I often just use polyester resin for mine. To make the above gems, I used the following materials: Castin Craft clear Polyester Resin, Castin Craft transparent resin dye (in the colors blue and red), latex gloves, a measuring cup with popscicle stick (for stirring), Castin Craft mold release, Easy Mold Silicone putty (for the positive) and glass chandelier crystals (or anything that resembles the specific shape you need). Oh, and some newspaper or cardboard to protect your work surface. Since polyester resin emits very strong fumes, you'll definitely want to do this in a well-ventillated area, or even outside! Wearing a respirator mask is also advised.
For your positive (the shape you want to duplicate) you can use just aboout anything, such as wood, plastic, glass or clay. Many cosplayers will shape the positive from modeling clay and impress them into a silicone mold.
For the above Zelda Twilight Princess crystals, I used glass crystals purchased from an online chandelier store to use for my positive and impressed them into the silicone molds. Since these gems will be front-facing, it doesn't really matter how the back looks, therefore, you can cover the backs with cardstock paper or aluminum foil if desired. I find doing so makes the gems appear more cloudy, so I don't add any backing to mine.
After I impressed my positives into the silicone, it took about 25 minutes to set.
Once your mold is ready, spray some of the mold release agent into the cavities and use a small brush (such as a painbrush) to spread it around.
It's now time to add the resin. Regardless of what type of casting material you use, be sure to read the instructions thoroughly to know how much of each part (resin and catalyst) you'll need to mix.
Simply pour the directed amounts into your measuring cup, use the stirring stick to constantly mix the agents together, for at least a minute, making sure to scrape the sides and bottom. After the agents have been well-stirred, it's time to add your coloring, if needed. Add only one drop at a time, stiring between each, to ensure the color doesn't get too dark too fast.
When everything's well-blended, slowly pour the mixture into the silicone mold, being careful not to let it spill over. At this point, you'll need to take care of any unslightly bubbles that may arise, before the resin cures. There are a few ways to do this. I usually place my resin under a desk lamp, since the heat encourages bubbles to rise to the top. Then I use a toothpick to work them to the edge and get them to pop. People have also used hairdryers and blown the bubbles around with straws. Just be careful not to apply to much air at once or your resin will spill over.
Depending on how large your gemstone is, it can take anywhere from 4 to 6 hours for the resin to fully cure. You can use a toothpick to test its firmness, If it passes the test, then bend back the sides of your silicone mold to let the piece fall out, or else gently pry it out.
At this point, you can trim off any excess resin using scissors or sand paper. Sanding is also recommended for removing small surface scratches and bubbles that you didn't catch. You could use a buffer tip on a dremel to polish it up, use some Flitz polish to combat cloudiness, or use some Castin Craft Resin spray (which is my preference).
Edit: After writing this tutorial, I experimented with EZ Craft and found that I like it better than Castin' Craft since it emits less odor and can be measured in equal parts (resin and hardener). The only thing I don't like better is that the final product is not as clear as Castin' Craft.