Can the roll to pick up the M'kraan crystal be rerolled? Nothing on the card prevents it.

You can't reroll relic rolls, but you can try again as many time as you want per turn per character.

2014 rulebook page 20-21:

If a character occupies the same square as a relic, that character may be given a power action to roll a d6 that can’t be rerolled. This roll is called a relic roll.

And to note, all special objects that include rolls to use/activate them have been updated to be called Relics so even Thor's Hammer and all of the older ones have this rule retroactively applied to them (even though it's not listed on their card).