Wood. The first thing is that we have to understand what wood is. In some ways, its a simple answer. Its cellulose. But that hides the the bigger picture. Lets start with something knife makers care a lot about.
Strength. What makes one wood stronger than another? The thing to know is that all wood is made of the two same compounds, cellulose and lignin. The best way to think about wood is as bundles of straws glues together. The straws are cellulose and the glue is lignin. You cant get stronger straws or stronger glue, its all the same. So how do you make it stronger?
There are 3 main ways. The first thing you can do is add more glue. Slather the straws in glue, fill every gap with it and fill in the straw interior with it. I will touch on the difference between heartwood and sapwood later, but this is the main difference in strength between heartwood and sapwood. A wood like this is something like paduak. The grain of paduak is not very different from something like maple, but the wood is good bit stronger and heavier because there is so much additional lignin.
The other two revolve around the cellulose, or in this case the straws. For one, you can make the walls of the straws thicker. This is what separates something like Oak from say, Alder. Oak has a very coarse, dry grain, but is still very strong. Each of the straws in this case are very thick, so the wood as a whole is stronger.
The other option is to make the whole straw smaller. Think about it as a box of coffee straws vs a box of drinking straws. The tiny coffee straws have more total plastic because each is smaller and they pack together better. This is the best for knife makers "Combined with the other factors" that yields the tough, durable and fine finishing hardwood we love, woods like rosewood and desert ironwood come from. By shrinking pore size you can get a fine finish. You cant tell the difference very well between oak at say, 220, 500 and 1000 grit finish, but the same sanding is easily distinguished in fine grained wood like ironwood.
Next is strength based on grain Wood is what is known as an anisotropic material. That is to say the strength of wood is dependent on the direction of the force applied. For insane, its not very hard to crush wood with the grain. That is to say from the side. But if you apply force down to the end grain of wood it is INCREDIBLY strong. Think about how a tree is strong. It supports vertical weight "The weight of itself" and does not require very much horizontal strength past the ability to resist wind.
It's well known that end grain is inherently weaker than face or side grain. But why? Think back to the straws glues together. If you have a 1 foot cube of straws, you have 4 faces that are identical. These are the 4 faces that show sides of the straws. You have two faces that show the ends. This is the same as a cube of wood. If you wanted to support weight with your straw block, how would you orientate it? You would have the "grain" running up and down! Because end grain is very strong in compression. But, what if we took a 1 inch thick slice of end grain. Now we have a sheet 1 inch thick and 1 foot square with the two large faces showing end grain. This piece could be easily snapped in half in your hands! End grain has very little strength against what is known as a bending moment, or simply being bent. Why is it so much weaker in this direction? Think about the straws. Each straw is glued to the one next to it by a short 1 inch long strip of glue! Because the grain of wood is only connected to other grains along the long axis and NOT across end grain, it has no resistant to bending but extreme resistant to compression like for a wooden mallet or and end grain cutting board.
How does this help a knife maker? Well it tells you how to pick your grain, but also how to use end grain if you do use it. What you need to do is make sure the wood experiences NO bending. THe best way to do this is to make sure the wood is dead flat before it is glued to the handle. Any gap will allow the wood to bend and large cracks to occur.