Oops, sorry if I'm late, but 会心 means meeting/after ones heart, while I think 丹 comes from earth or mud; probably 土丹 meaning mudstone considering the marble thing.
I'd probably translate them as 'spirit focusing clay/stone' if I was trying to be accurate, but for the sake of making sense 'focus pebble/stone' would be my translation.
Alternately Heart/Spirit Pebble(Clay)/Stone, depends what fits the rest of the translation best.
Also you have your marbles confused, mrbl1 doesn't work in dragon form, mrbl2 does; just as king mike said (Thus my pebble > stone thing).
Aeana you are correct it does appear as critical hit in several games; but the it doesn't directly translate. 会心の一撃 (kaishinnoichigeki) translates as a blow/strike meeting ones heart (doesn't make too much sense in English does it?), so strike is really not a good translation, at least not directly, although it does convey the use of the item quite well.
Hope I was of help