The worlds of fantasy and science fiction are filled with all kinds of precious crystals. The Sorcerer's/Philosopher's Stone of Harry Potter. Star Trek's dilithium crystals. Lovecraft's Shining Trapezohedron. Tolkien's Arkenstone. Traveller's zucchai crystals.*
From a presentation standpoint crystals are awesome. They're visually striking and immediately convey an impression of value and uniqueness. They get attention, and the bigger they are the more appeal they have.
Unfortunately, the larger a crystal is the more it costs. That applies to both the real and faux kind. Want a real crystal ball bigger than three inches? Get ready to pay through the nose. Get the same thing in cast acrylic and your wallet won't be screaming quite so loudly, but you'll still be laying out some cash.
A cheaper alternative is to build your own prop crystals. If you absolutely, positively need something huge you can invest in casting one from clear resin or having it machined from acrylic stock. The most inexpensive option is building up larger crystals using "table scatter" gems, the cast acrylic decorations beloved by wedding and event planners, held together with hot glue. They're readily available at big box stores in the craft section in a limited range of colors, but you'll find them in every color of the rainbow online.
For this project I used these red acrylic gems from Amazon. A two pound bag is just $7, with free shipping if you're signed up for Amazon Prime. That's the lowest price I could find from a domestic supplier, but you can order huge amounts of these things from Chinese suppliers.
Once you have the gems in hand you're almost ready to go. Wash down the table scatter with some warm soapy water to remove any mold release. While the gems are drying, get your glue gun warmed up.
The actual assembly of the crystal is pretty straightforward. Once the gems are totally dry, just start sticking them together with hot glue. What's impossible to convey in words or pictures is that you'll be putting together a three dimensional puzzle.
Ideally, you want to build up the shape by fitting together the gems with limited gaps. That's going to assure that the finished crystal has a high optical clarity, diffusing light throughout it's structure to duplicate the translucent appearance of a real crystal. If there are any major voids don't be shy about filling them up with the molten glue. I used a high-temperature glue gun. That provided excellent adhesion to the acrylic, but had the drawback of requiring significant cool-down time for the glue to solidify. The last thing you want to do is accidentally stick your finger into molten (370 degrees F) EVA plastic. Ouch.
Here's what I had after about an hour of work, including all those breaks for cooling.
Not too shabby. The fist sized crystal used up roughly half of the two pound bag. As you can see, it has excellent light transmission qualities. Those dark spots? Reflections of the black velvet backdrop that's off camera. They're getting bent around by the internal facets of the crystal mass. Because of the high refractive index I think this is going to look stunning when it's underlit. On that front, I've ordered a few puck lights to experiment with.
Another nice aspect of the assembled crystal is that it has a decent heft to it. That's not important if you're just looking for a display piece, but it's a desirable feature in a prop that's going to be handled. Package it up in a storage box and you're going to have a very cool plot token.
Here's a comparison shot of the finished crystal with a glass deck prism and a quarter for scale. The acrylic transmits a lot more light, while the glass has more color depth.
For roughly $5 in materials this is a great little prop. The one change I'm going to make in the next iteration is to use a clear rubber ball as the core of the crystal. The current version can take a goodly amount of abuse without damage, but adding some flexibility should make it even more survivable.
*Ol' skool RPG callback, yo! Sweet Jebus, I just realized I've been a citizen of the Imperium for close to 40 years now.