I've got a couple of methods for you.
1 - The traditional way.
Mix 0.5oz of silver nitrate into 20oz of water, apply to surface and then overcoat with 2 coats of Hydrochloric Acid.
Set aside and allow to dry perfectly and evenly by placing in a darkened room.
Finally wash the entire surface with Ammonia.
2. Alternatively, I know of a company that mixed Black,White and Teak Water stain together to create the effect.
Hope this helps
WM