Hold seed in Vice Grips and use Dremel tool with grinding wheel. Works nearly all the time.
Some very hard seeds - soak in Stump removal solution for 2 - 4 hours and then do above.
If you still have a hard time, do all of above and use a small triangular shaped tool to create the indentation where you can use the Dremel tool.
Used the above methods on seed of African Dream Root seeds which ARE impervious and hard as Titanium and make canna seed seem like child's play.
