HBP Spinner's End said:'That was not my fault!' said Bellatrix, flushing. 'The Dark Lord has, in the past, entrusted me with his most precious - if Lucius hadn't -'
The Dark Lords most precious - most precious what?
Notice Bellatrix never did finish that statement - but what could the Dark Lord have trusted her with what she considered him to hold precious?
We know that Voldemort doesnt hold that many things precious - so what could it be, could he have trusted her enough to let her know about his horcruxes or something of equal value to him?
There is no doubts that she really is every loyal to Voldemort and there are not that many in his group he can say that about!