I'm pretty sure any hardwood is suitable to use in a tank, except evergreen because of the sap (I seem to recall reading that somewhere). It has to be dried (not green/straight from the tree), driftwood from the beach or bog wood from a creek/lake/etc. It has to be collected from somewhere that has no contaminates (toxic run off/sewage/weed killer) that could leach in to the wood and end up in your tank.
When preparing the wood you need to scrub all the dirt and loose bark off. Then soak it in a tub of water to leach all or most of the tannins out and to water log it. Using hot water will help speed up this process. The water will need to be changed every day or so. This may take weeks or months to achieve. If the wood was collected, instead of bought from a LFS, you will need to kill all the nasties that may be on/in it. Some people use bleach but I don't like the idea of that going into my tank. You can boil it in a large pot for a couple of hours, rely on the hot water you are soaking it in or my favorite, put it though the dishwasher
(no soap of course!).
After all this, if it still doesn't want to sink, you can use a stainless steel screw to fix a piece of slate to the bottom that you can hide in the substrate. Or just sit a stone on the top.
Wow, I typed way too much lol. Hope it helps!