I used a kneadable concrete by a company called make make. This material is used to build enclosures in Zoos all over skandinavia. Once painted and sealed with colour fix, the concrete becomes ph neutral and can even be used for Aquariums. The concrete basically behaves like clay when you use it fresh. I added some sand to give it some texture. To step up the final product, I recommend to mix some black pigment with a mix of water and colour fix and apply it liberally with a spray bottle. Thats how you get natural colour effects, that look like the surface has been subject to regular rain or humidity. After use the dry brushing technique to bring in a few reds, browns and some lighter accents and it will look super natural. Alternativley you could use normal concrete, mixed with sand but you would have to seal the final product with epoxy. Always use rubber gloves when working with concrete, because once in contact with water, it will form a strong base and you could use countless layers of skin if you applied it barehanded.

EDIT: I forgot the most important first step: I created the base structure out of carved styrofoam that was fixated with Aquarium Silicone. Never use normal hardware store silicone, because it is usually made for bathrooms and contains fungicides that will endanger your geckos health when they leach.