The periodic table lists all the elements in order of the number of protons they
have. Every time you add a proton to an atom, you get a new element. Every element has been discovered from 1 proton (Hydrogen) up to 118 protons (Ununoctium), so there are no more gaps to fill in. When the periodic table was published in the 1800s there were many gaps that have been filled in over time. However, we can still find new elements that have higher numbers of protons: element 117, ununseptium, was only discovered this year, and element 118, ununoctium, was discovered in 2006. The search is now on for element 119. Unfortunately, atoms with this many protons are very unstable and tend to break up into smaller atoms, so they can only exist for very small amounts of time. You’d never be able to get a ring made of ununoctium.
As for other substances, most of them are in fact compounds (a mix of different elements, so for example water is a mix of the elements hydrogen and oxygen). We are always finding new ways to mix different elements together to get new and useful materials – that’s the job of chemists.