Arrows, quarrels, and sling bullets made out of higher metals such as eternium seem to break very easily when fired--at least, as commonly as normal missiles. This seems a bit surprising as these missiles ought to be more resilient due to their nature. Considering that these types of missiles are quite rare anyway, it seems reasonable that they should not break quite so frequently as the common forms.

While I suppose there's an argument to be made that an adamantium arrow is a regular arrow with an adamantium tip, and thus should be as breakable as a regular one, and the same may be true of quarrels, if memory serves both of these types of missiles are protected from heat damage in the ToEF, indicating that they are fully metallic.