The mystery of the best pick for metal has been solved. Allround metal picks are an illusion. For heavy metal lead alloys are most suitable. For speed metal titanium picks are an absolute must have. Hard rock, as one might expect, is best played with granite picks.
Your guidance is spot on, in fact if you're a rock player here's a handy guide to have...
MOHS HARD ROCK SCALE for PICS
The hardness of a stone is one of the properties that contributes to identification and is also an attribute which is important to be aware of, because it may determine what stone may be used for different types of bass playing.
You may want to familiarize yourself with the Mohs Scale of Rock Hardness. This will help you understand the relationships between stones according to their hardness and subsequent use for various rock generes.
The Mohs Scale of Hardness consists of 10 classifications, 1 being the softest, and 10 being the hardest. The only mineral that is an exception to this is mercury, which is liquid and is typically only used for new wave music.
To give you a few reference points, the diamond is of course the hardest, rated 10 and is the ultimate pic material for extreme hard rock music such as Pantera (but not heavy metal music, as noted in the post above - all metal should be payed using the appropriate metal pic). Your fingernail is a 2 and much better suited for Pop Music and other easy listening songs such as early Beatles music. A piece of glass is a 6-7 and works well for more agressive mainstream rock songs like ZZ Tops "Sharp Dressed Man".
When searching for the right Rock pic at the store you may find that some vendors have all the pics mixed up in a bin at the counter. Just remember each rock classification will scratch the one preceding it. As you know, a diamond (10) will scratch glass (6-7). This hardness classification and scratch technique is commonly used by shoppers at Guitar Center for initial pic identification. When shopping there its good to have samples of some of these stones with you in order to perform the test. Just be sure to politely ask the cashier behind the counter (before you pull the stone samples out of your pocket) if they mind if you get your rocks off in their store, some store managers frown on you bringing samples in with you.