Quote:
Originally Posted by
Ameh 
Do we have canon (SS or otherwise) for what the hourglasses look like in the negatives? Heather and I were talking about it and we both agree that black crystals that disappear as points get added would make sense, but I can't remember if we ever had like, an actual protocol for it.
According to Harry Potter Wiki:
When points are earned, some gems (depending on house colours) fall from the upper bulb into the lower. When points have been deducted, they do the opposite, retreating into the upper bulb.
It also says:
In Harry Potter and the Half-Blood Prince, when Severus Snape takes points from Harry after meeting him at the Hogwarts gates, he remarks that Gryffindor would be starting out in negative points. However, previously in Harry Potter and the Order of the Phoenix, Snape was unable to take any more points from Gryffindor when they had none left. McGonagall had to add points before any more could be removed.
That leads me to believe you can't technically be in the negatives, as you have to have points to lose them.