Not sure if this will help, but I've collected jewelry box clams and the ones from the atlantic are usually not that big, most are under 2-3 inch and yellow or bright orange, so they stand out vividy from the live rock they are attached to. I guess my question is are you sure it's a jewelry box clam? It could be any of the thorny oysters that come from the pacific or atlantic.
As for care, they are hard but not impossible, they have a better survival rate then flame scallops, as long as you feed them. I use a mix of food for mine, I have both jewelry box and thorny oysters, what I feed is zooplanktons-L from brightwell aquatics and mix that with vitimens, coral frenzy powder, and roti-feast by reef nutrition. I also add coral vitimins from H2O aquatics. I feed this everyday as I have a lot of non photosynthetic octocorals and christmas tree worms. With this food I've kept my animals for almost 2 years now and my octocorals have not only doubled in size, but have spred to new rocks and are sending up new branches that have become whole animals, not just buds.
With this much food, I have a huge nitrate problem (other values are perfect) but I use natural sea water so I do a lot of water changes and the tank health stays pretty good. Just feed as much as you can while maintaining water quality and it should live fine. As far as it dying, it will not open and if it stays closed you should be able to pull the shell apart with no resistance. Just watch the water values, if it dies you may see a spike in ammonia and nitrite just as if a fish were to die. Just pay attention to the tank and watch you will know if it is a problem.
Hope that helped, and good luck
