Are adamantane crystals covalently bonded?
Adamantane is a molecular compound, much like naphthalene, paraffin, and many, many others. Molecular compounds consist of molecules, which stay pretty much the same in the gas and solid states. A molecule has certain molecular formula. Each carbon in a molecule is linked to a certain number of hydrogens and other carbons. There are no dangling covalent bonds to be formed between molecules, let alone between crystals. Consequently, you can't just take some adamantanes and covalently fuse them; some people would discuss diamond crystal structure in such terms, but only in a metaphorical sense.

Adamantane molecules in a crystal (being non-polar molecules without any specific interactions) are held together only by the weak van der Waals forces. Because of that, yes, the crystal is not mechanically strong at all (again, think of paraffin wax).