In general, hardwoods come from trees that produce seeds that have a coating in the form of a fruit or a shell. Softwoods trees by contrast do not produce seeds that have a coating, they produce seeds that are dropped to the ground as in the case of trees that produce cones such as connifers, making it easier to propagate. Softwood trees usually have needles and cones.
Hardwood trees typically take longer to grow, creating dense woods that are likelier to split when nailed. Softwood trees grow much faster and produce lumber does not split as easily.
The Janka Scale of Hardness is the most common method for testing and communicating the hardness of wood. It uses a number system to describe the wood and how much of pounds-force (lbf) is required to imbed a .444″ (11.28 mm) diameter steel ball into the wood to half the ball's diameter.
Some woods that are considered hardwood, such as Balsa (90 lbf) and Paulownia (260 lbf) are actually quite soft, in fact, softer than softwoods. This just goes to show that the terms softwood and hardwood technically describe the type of tree; connifers vs angiosperms (trees that produce seeds that have a coating).
The hardest commercially available hardwood is hickory, and it is five times harder than aspen, one of the "soft" hardwoods.
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