perfect skulls are perfect gems. Problem is that some/most bnetters don't know this and thus don't take them. If you manage to sell them, there is no reason why they would be worth more or less then the normal pgm.
If you want, you can use them to reroll rare items - jewels for lld or tiara's are used. Or you can reroll magic items like with normal pgms - grand charms.
P.S. note that only topaz, diamond and perhaps emerald - and skulls - are assumed to be one pgms as they aren't used in crafting. Amethyst, rubies and sapphires are used in crafting and thus some value them higher.