There are 2 types of lignum vitae. Palo Santo, which is lighter in colour (usually cream sapwood and greeny heartwood) and lower in density, a Guaiacum Officinale, which is darker (cremay sapwood again but usually brown or darker heartwood) and denser.
G. Officianle is known as true Lignum Vitae, and is what is used for crown green bowls. However, it is on a CITES watch list, which means that any G Officinale trees cannot be cut down without good reason, therefore true lignum vitae on the market is limited to what has either been cut downpreviously, what has already been made from the wood, or the small amount of trees that arre approved for felling each year
Palo Santo is much easier to get hold of, but there are other woods available that are denser and harder