diamond back rattle snake

Question
QUESTION: Why do some of the diamonds appear to be heart shaped?

ANSWER: Every individual animal has an individual pattern, which is the result of a combination of genetics and development. It's not unusual for patterns to be 'less than perfect', and certainly the term 'diamondback' is a human perception of the usual shape for the animal's markings... not a hard and fast rule for them. :)

Although, if you prefer a more flippant answer, it is because he loves you.

---------- FOLLOW-UP ----------

QUESTION: So that does not mean that they are female?

Answer
No, it doesn't mean they are female.  
This is how you sex a diamondback rattlesnake:  http://www.youtube.com/watch?v=09e9SxfndZs

What the man is doing, is inserting a small rod called a sexing probe into the snake's cloaca, in the direction of its tail.  In a female, the rod will only go in a short distance.  In a male, it will go deeply into the tail, into one of the pockets in which the snake's sex organs are stored.  (Male snakes have a pair of sex organs called hemipenes, which are kept inverted inside the tail when not in use).
There is no way to accurately tell their gender by any external characteristics.

There are some species of snakes which are sexually dimorphic (males and females have external differences, so you can tell them apart if you know what to look for - for example, in some, males have spurs while females don't, or the male's tail is significantly longer than the female's - but in many species, there are no outward differences).

In any cases where there is a question, an experienced person can correctly determine the gender using sexing probes.  In some species, pressure can be applied to the tail to force the hemipenes to emerge (called 'popping'); this should also be done only by someone who is experienced, so as not to injure the snake.  The species I breed, ball pythons, can be sexed via popping or probing, but there are no outward differences.  (Both sexes have spurs, and tail lengths vary).  The common garter snake that people are most familiar with can usually be sexed by tail length.  Males are significantly smaller than females, and have much longer tails.