Magnetite Stone (Banded Iron Formation)
Precambrian era (1.4 to 2.7 billion years ago)
North America, Western United States
Dimensions: 3" x 2" x .5", weight not given

Magnetite is fromed of oxides of iron, and is ferrimagnetic. You are correct in identifying this example as a pseudomorph. It is a transition of a mineral from one to another and is a category of mineral. They develop through complex geological processes where the chemistry or structure of an original crystal changes, but its outward appearance is preserved. Petrified Wood is a good example. Over time, the original wood tissue is completely dissolved and replaced by silica, quartz, preserving the exact shape and grain of the wood.

This example could have had iron rich silica deposits that transitioned the wood, or this could be pure magnetite that formed and pressed against a natural wood in a magma state and left an impression. It could also be the pressure and layering of the development of the example that gives it the unique bark-like formation.

Be mindful in sales of items that are found on government lands and the protections and laws surrounding their resale.

Previous:
To give the best assessment of your example I have a few questions to help narrow down the possibilities.
Where did you discover the stone: geographical location and in what type of environment (ie a forest, a river bed).
Was it in the current condition? Di you clean or remove debris or polish the stone?
Let me know and I'll write the assessment for you.