From the campaign:
The material we now call "Stones" is made out of a Polymer-Plastic. Although the initial mold/production cost is very high, the end result is a product which is everything we wanted - very durable and highly customizable, which means they can be snipped, cut, heated and altered, and can take a beating! They will be produced in a gray-black material with built-in primer, which means all you have to do is take them out of the box, wash and paint - very simple. Also being very durable means easy to store. Just slide them into a duffel bag or box. Most importantly, being affordable means you can get more for your money, so you can paint one set stone grey and do another with a sand color. You can also customize them or even use them in other games as sidewalks, courtyards, etc. The possibilities are truly as boundless as your imagination!
One last thing about these amazing gaming tiles is that we added an "anti-slip" backing to them. That means that when tables are bumped, the tiles are less likely to slip and slide, especially when you play them on a cloth mat - it simply grabs hold of the material and in most cases stays where you placed it. We are intending to offer a cloth mat to be used with the tiles later in the Kickstarter, so stay tuned.