Daren is correct. I have 4-5 different colored pieces of rosewood drying in heat box right now. They come from all over the world.
There are many things you can do to it but here is what I usually do to an old fingerboard.
First I would clean it. I use a very diluted ivory soap & water mix. I flip the guitar face down so the water doesn't wick down into the grain so fast. Doing around 5 frets at a time, dip a sm. scrub brush into the soapy water mix scrubbing away at a block of frets at a time. After doing a block of frets, you need to clear off the wet muck. I then take a clean paper towel and absorb the water before it starts to soak in. Then I move onto the next five frets. After being finished cleaning it with the brush, I take a damp paper towel and clean the entire fingerboard. From there, I take an old t-shirt and put a little bit of mineral spirits on it and wipe the entire board down again. Use a light colored shirt so you can be sure you are still removing grime. When nothing comes off, your done. From there, I let it dry over night. Sometimes I place it aiming toward the sun for 30min.
The next day you can do a few things to it but I would experiment on a few frets first. Most likely I would linseed oil it. I never tried lemon oil but I'm sure it would work. Grape seed oil is another. Let it sit 5min. between coats and hit it again. I would apply coats 3-4 times then wipe it off with a clean paper towel.