Let's say my patent uses the properties of a material that's not yet developed or that has being developed but it is still being tested. How is it patentable ?
There are two concepts that will help here:
First, as they say, "The applicant can be their own lexicographer." This means that you can define your material however you want. You are literally able to describe a material with certain properties and give it a name in the specification and then use that name in the claims of the patent. Plain meaning will obviously have bearing on the courts' and patent office's interpretation of any term you use, but you are also able to define terms to describe your invention.
Second, a major requirement of a patent is that it must be enabling. This means that a "person having skill in the art" could, without undue experimentation, use the patent to actually make and use the invention. If a material that does not exist is required to make the invention, then the patent would not be enabling. With that said, a patent on something that is impossible to make would not be of much value in the first place because it would also be impossible to infringe. However, if the material is still experimental but does exist, then the patent would be enabling.