In an earlier post, we discussed other precious metals besides “the big four” (gold, silver, platinum, palladium), including osmium and ruthenium. Here, we continue the discussion with two more precious metals, along with an incredibly common one that is precious no more.
Rhodium: If you are a jeweler, you’re probably more familiar with rhodium than most people. Actually, rhodium is far too rare and valuable (worth slightly more than platinum) to be the principle element in any piece of jewelry. However, it is quite durable and uncannily shiny – which makes it an excellent coating material. Quality white-gold jewelry is often rhodium plated to increase its durability and luster. Sterling silver is also rhodium plated to increase its resistance to tarnish. Additionally, rhodium is also an important part of catalytic convertors (similar to palladium). Because rhodium is so rare, it has a high recycling rate – the bulk of which comes from old catalytic convertors.
Iridium: Osmium narrowly beats iridium for the title of densest metal. However, iridium takes the crown for the most corrosion resistant metal. Although there are ways to break it down (as a mater of fact, iridium is extremely brittle), it is totally resistant to all kinds of acid. Any industrial application that needs heavy-duty corrosion resistance will use a little bit of iridium alloy. Famously, iridium is used to explain the extinction of the dinosaurs. While rare on earth, iridium is found in abundance in asteroids. However, there is a stratum of clay throughout the entire earth’s crust that scientists use to mark the border between the Cretaceous and Tertiary periods of geological time. This clay is rich in iridium dust. In theory, this layer of iridium occurred when the impact of a massive asteroid or comet collided with the earth creating a massive dust cloud containing iridium. The cloud would have spread across the world, depositing the iridium and blotting out the sun, killing the dinosaurs.
Aluminum: That’s right – prior to the 20th century, aluminum was a precious metal worth more than gold. Although aluminum is one the most commonly occurring elements in the earth’s crust, it never appears in its pure form. Refining pure aluminum used to be very difficult and very costly. These factors elevated it to the status of “precious metal.” In the late 19th century, the Hall-Héroult process was invented and gave the world a cheap and easy way to refine pure aluminum. This combined with aluminum’s commonality reduced its value and eliminated its precious metal status.
Knowing that there are other precious metals out there and seeing what happened with aluminum all those years ago begs the question: what will we be refining tomorrow? Right now, it looks like gold, silver, platinum, and palladium are here to stay for a long while. But, as science continues to advance, who knows what the distant future holds.