Maybe an easier way to answer this question is to tell you what base metal is not. It isn’t precious metal (gold, platinum, palladium and silver). Common examples of base metals include copper, zinc, nickel, and aluminum. Base metals can be alloyed, meaning a metal is made of a mixture of some of these metals. An example is tin and copper mixed in a certain ratio to make bronze.
Precious metal, as the name implies, is more precious. This means these metals are not as commonly found as base metals. The fact that base metals are a bit more plentiful means that they aren’t as expensive as precious metals. Therefore, working with base metals is a more economical way to make new designs. Sometimes I make parts of a design, or a whole design, in base metal in case I make a mistake or just don’t like the design.
Another reason to use base metals is that they provide a different look. For example, copper provides a unique color that gold and white metals can’t provide.
Fun fact: Rose gold looks so rosy because it is gold with a high copper content. More information about gold and alloys here.
Base metal is often used in jewelry and plated. For example: A design is produced in brass and then plated in gold or silver. This is more economical than making the piece entirely in gold. Of course, this plating can wear off the brass after you've worn the piece for a while. There are also some negative impacts on the environment from mass produced plating. As always there are trade offs when balancing what's best for the environment and how we adorn ourselves. For a deeper dive into plating, here are couple of post about it: Plating vs. Gold Fill and Rhodium Plating.
I don't want to leave the impression that base metals are bad or that jewelry made from them should be cheap. The jewelry can be gorgeous, it can actually be less harmful to the environment and the end product can cost quite a bit of money.
As far as environmental impact goes, I, like many makers, appreciate upcycling materials. I'm working on a collection of jewelry made from vintage tins. I also have a vast collection of keys and a jar full of spent bullet casings. I tinker with all these materials (made of base metals) to make jewelry.
Creativity is valuable. In other words, what the artist has done with materials to transform them into art is valuable.
I write these posts to give you a better understanding of the jewelry making process and materials. Hopefully this helps you choose what to buy and know what you’re buying.