Ores are rare because the elements that make them up are relatively rare. Most of the earth is made up of oxygen (O), silicon (Si), magnesium (Mg) and iron (Fe). The Fe and Mg combines with Si and O to make minerals like olivine and garnet .

Now the elements like say nickel (Ni) or gold (Au) or chromium (Cr) are present in far far smaller concentrations. So, in order to make a valuable ore deposit there has to be a way for these rare elements to get concentrated. It does happen but it takes special conditions. Geologists study these conditions.

You should start a rock collection and begin to learn about Geology and Mineralogy.

This is a really great question that not many people think about! As you know, an ore is a rock or a mineral that contains a large amount of some valuable element (I will use the example of gold) compared to most other rocks, which contain little or none of that element.

The cool thing is, that the element itself is often spread out all through the earth, but in such vanishingly small quantities, that it would be impossible to mine it in an economic way (i.e. there is so little, that the cost to try to extract it from the rock is much more than the value of that small amount of valuable element). For example, the earth’s crust, on average, contains just 0.00000031% gold (which, as you know, is a valuable element). That is such a small fraction of your average rock, its not worth going after in just your average rock.

But an ore – a rock that contains enough gold that it is worth going after – is formed when various earth processes naturally collect that vanishingly small amount of gold from a large area of average rocks, and then concentrate it in a single location! This is what makes ores rare – because the processes that do this are rare!

For example, one process that can concentrate gold to form an ore is the circulation of hot fluids through the earth’s crust (we call them hydrothermal fluids – they may consist of water and gasses released from magma deep in the earth, or these fluids may come from groundwater that has been heated (perhaps by magma) in the crust and caused to circulate through large expanses of rock). These hot fluids are good at dissolving that tiny amount of gold out of the average rocks they travel through, collecting it and carrying it to a new location, and then when the fluid cools, depositing all the gold it collected in one spot. Because heat is necessary for these fluids to dissolve and collect the gold, this process often occurs deep under volcanoes, where you are likely to have magma as a source of heat.

And then of course, we must also have the lucky chance that one day those ore rocks, formed deep under a volcano, might exposed at the earth’s surface, which can take millions of years.

So basically, these processes that collect and concentrate our gold occur only in rare places, and under rare circumstances making the ores they create a rare commodity.

I think one other interesting thing to note is that whether a rock is considered an ore depends on how good our technology is to extract the valuable elements from the rock at a low cost! So in the future, when we continue to improve our technology and can get more gold out or an ore rock at a lower cost, some rocks that have just to little gold to make it worth it to extract today, may become economically viable ores in the future, when we can get that gold out for cheaper! Likewise, if the price of gold goes up, rocks with less gold in them may become ores simply because that smaller amount of gold is now more valuable to people! So it is sort of an ever changing thing, which I think is pretty neat.

To answer your question we need to go back to the time before the earth formed. Stars which eventually died formed the material our planet is made out of. Nuclear processes which make elements like gold (Au) and platinum (Pt) tend not to produce as much of those elements and are made as stars die. Some researchers even suggested recently they can be formed when certain types of black holes are created. So the building blocks of Earth already did not have as many elements which form ores.

If you pick up a rock that's an ore then often you will notice that it's heavy relative to a rock of the same size that doesn't have ore. That's because the minerals in ores are often heavier as well. When the earth formed many of the elements which we look for in ores settled into the earth's core. Often only unusual geological processes concentrate these elements enough that it is worthwhile to mine them.

Some types of ore aren't as rare. For instance, there are over 2 trillion tons of iron ore in deposits worldwide.

Some elements are more common than others. This ultimately goes back to astronomy where stars make more of some elements than others, but on Earth different elements behave differently. For example, the earth has a metal core made of iron and nickel, but both of these metals are much rarer on the surface. You need special conditions and chemistry to accumulate these and other metals.

Ores are so rare because they contain a really high abundance of elements that are very uncommon in our earth. If you look at the periodic table, you'll notice that it is designed so that the most abundant elements in the earth are in the top rows, and the least abundant elements are in the bottom rows. So elements like the Lanthanides (we call these the Rare Earth Elements) and some of the transition metals and post-transition metals (like the ore-forming elements Ag, Au, Cd, In, etc.) are very rare in our earth.

For example, when you pick up a typical rock on the ground, that rock will usually contain less than 1 part per million (ppm) of the rare element silver (Ag). Ppm is a type of ratio we use in Earth Science to describe the quantity of something. Imagine that rock is made up of a million atoms; only ONE atom in that group of 1 million is going to be an atom of Ag. And that's an overestimate since Ag is usually much less than 1 ppm. So if you want to mine silver for a profit, it wouldn't make much sense to take it out of normal rocks, because it's so scarce.

So it's really difficult to take a really rare element and concentrate it in a really high abundance in one space that we can access. We know that silver, for example, likes to bond with sulfur to make sulfide minerals. In a sulfide mineral, you can find around 1000 ppm of Ag instead of just 1 ppm. So geologists that work for mining companies look for sulfide minerals if they want to find a lot of silver. In some places, like in mountains that were formed at a subduction zone (like the Sierra Nevada), you can find a lot of Ag and other elements that also like to bond with sulfur. Geologists are still studying why places like subduction zones concentrate these very rare elements, because we still don't exactly know why it happens.

Some geologists think that when a magma forms at a subduction zone, the sulfide minerals in the magma sink to the bottom of the magma chamber because they're so dense. These sulfide minerals get stuck at the bottom of the Earth's crust. Then, later on, a different type of magma may come along and melt those sulfide minerals at the base of the crust, and carry all of the rare, sulfur-loving elements like Ag along with it to the top of the crust where they form ore deposits.