I have two little black lines in two different nails. they are really small and look like splinters. googled and found that could be from heart.

"Could" is the key: term because this is not most common. Look up splinter hemorrhages using both Google images for many more images for comparison & on Google for many possible basis for these. Like most physical signs these can result from many different issues. Better diagnosis is always about recognizing multiple inter-related patterns; not just individual symptoms & signs, each of which are usually non-specific.