As is usually the case with a biomedical engineer, I know enough about what you want to know where to look (and not a lot more). What you really want is a materials scientist.
Some time back I had two physics PhD candidates as students in my UVa karate club. They did a project where they worked with a local artisan to make traditional bos out of various available types of wood. But before they touched the first piece of wood, they went to the books and did their homework. They came back with a chart that had each wood as a row variable and the physical properties of the wood as a column variable. Then they made their orders, and had several bos made of each type of wood. As I can recall, they used the following types of wood: oak, hickory, wallnut, teac, cherry, mahogony, ironwood, purpleheart, greenheart, etc.
After all the work was done, our next step was to unleash our kobudo folks on the bos. We had some prearranged kumite that we were working on. Some of it involved some rather hard-style blocks. Some of the more dramatically powerful blocks used whole-body power in both attack and defense.
The thing I remember most vividly is how wood that we normally think of as "hard" ended up being so brittle. The walnut, mahogony, teac, etc shattered with only a few blocks from our boistrous group. Some of the others lasted a bit longer.
The winners? The number one wood, in my opinion, was.....oak. Really! It had the right combination of level of hardness and flexibility. Purpleheart and greenheart were second because they maintained their surface integrity via hardness and yet did not shatter anywhere near as easily.
Bottom line...there are a host of different properties of materials to consider if you want to look for things that shatter easily. I'll talk to my buddies (one's a professor at UNC, the other works for Lucent I think) and see if I can get them to give me the list. Then maybe we can create a set of criteria that would determine if something has the smashing index you desire.
If anyone else can steer us there quicker, by all means do so.