Claims about asteroid mining talk about how many resources are on each asteroid, with claims of "trillions of dollars" of raw materials. Ignoring the cost of getting to the asteroid, have previous missions turned up evidence that these materials are actually minable? For instance, it may be true that there is lots of iron, but if that iron doesn't appear in deposits that can be easily processed and extracted it doesn't matter at all.
There have been no sample return missions from M-type (metallic) asteroids. Their composition has been estimated from spectroscopic data and radar albedo. The IR spectra of these asteroids was matched with meteorites showing similar spectra, and the asteroids were assumed to have the same composition as the meteorites. But meteorite spectral matching with asteroids is problematic, so chemical analysis of meteorites does not necessarily correspond to the composition of asteroids with similar spectra.
Using near-infrared spectroscopic analysis, this paper https://iopscience.iop.org/article/10.3847/PSJ/ac235f concluded that “the amounts of Fe, Ni, Co, and the platinum group metals present in 1986 DA (a near-Earth asteroid) could exceed the (Earth) reserves worldwide.”
However, they did not describe the ore mineral types. The mineral type is essential for assessing the challenges of extraction. For instance, nickel is usually present as nickel sulfides. On Earth, these are extracted by froth flotation. This requires water and gravity which are both in short supply on asteroids.
Iron ore is usually mined as iron oxide. A strong reducing agent (such as charcoal or coke) is needed to reduce iron ore to metallic iron. Typically, 630kg of coke is required to make a ton of steel. https://corsacoal.com/about-corsa/coal-in-steelmaking
Methods for refining ores on Earth have been optimized for Earth conditions. Water and coal are commonly used because they are readily available. In space, electrochemical reduction is a promising alternate technology.
Earth has a highly oxidizing atmosphere. Many oxide ores were formed by atmospheric oxidation of exposed rock in the 2.2 billion years that Earth has had an oxidizing atmosphere. The metal-rich asteroids have obviously not had the same exposure to oxygen.
Assay data is needed before buying asteroid mining stocks.
Iron? Maybe, but...
The first material being considered for mining (you may be surprised to find out) is water. The processes for mining water are generally well known and benign (ice has a very low crushing strength and high content of desirable volatiles) so it's not unreasonable to extract water from just about any form we can find it in. But there are also many other prospective mining feedstock options, with various chemical compositions and physical properties, including carbon, nitrogen, iron, nickel, sulfur, and platinum-group metals. The processes of mining any of the above may include drilling, blasting, cutting, and crushing.
Extraction may involve chemical or physical processes such as thermal decomposition of minerals and salts to release water vapor, Mond process chemistry, electrolysis and many more techniques.
Fabrication also may require heating, distillation, microwave sintering, or removing contaminants with other compounds.
Getting equipment to do all this up there really does add up. But it's not impossible. Just expensive.
However, the cost analysis of feasibility is done by comparing the cost of end-to-end retrieval of the material against its so-called "up-mass" (mass that needs to be lifted from Earth into high lunar orbit). The cost of returning an asteroid to the same high lunar orbit was estimated at \$2.6B (15 years ago, and perhaps under-estimated) and that'll get you a 7 m asteroid with an up-mass of roughly 500,000 kg. It was also estimated that each kg currently costs about \$100K. That's $50B. So if the asteroid mining technique only costs \$2.6B for the same amount of material, then by Grabthar's hammer, what a savings! right?
Note: I don't personally find this tantalizing because the Earth-market value of this material is still zero -- it's water. And the practical use of water in space-faring is in growing crops, keeping people alive, radiation shielding and making rocket fuel. But it doesn't directly have any business value. It's largely the government funding lunar or Martian exploration that would be the consumer of such water - trying to save money on lifting water from Earth to the moon. I'll bet my money on the Artemis mission's endeavour to extract ice from craters at the moon's south pole. Still - it costs a lot to get water to the moon, so if we can make use of in situ water, that's preferred! Now, if there were a way to get the material all the way back to Earth to be used by existing companies, and if we could get rarer metals, that would be amazing too! But I'm not yet aware of any feasible plans to do so.