We raised a question yesterday, brought up by Island Adventure. Unfortunately we have no answers for you: only speculation. It’s our lot in life, and it’s a good thing we enjoy it.
Yesterday we observed that the monster on the island was particularly easy to kill: a normal human girl with a sharp stick was able to puncture deep enough into its body to force it to retreat into its gem. We’ve seen that the Crystal Gems are capable of some pretty impressive feats, and can take hits to match. There is one time we’ve seen a regular Crystal Gem actually take sufficient damage to be forced to retreat to her gem: when Pearl was stabbed by her holographic projection.
It’s very interesting that both were killed by piercing wounds, but as we said yesterday we find it unlikely that this is a specific weakness to piercing attacks. Garnet’s fist will deal a lot more force per square inch than Sadie’s stick would, meaning that despite being much larger it would still penetrate more effectively. For there to be a weakness to piercing, it would have to be a particular vulnerability to interaction with small, fast-moving objects. This would make Gems vulnerable to arrows throughout history, making it very unlikely they could force their way onto Earth.
Then there’s also the fact that when the Crystal Gems go monster hunting, they use their own innate weapons rather than any sort of specialized weapons that would take advantage of their foe’s weakness. This may work fine for Pearl, who uses a spear, but Garnet would certainly use whatever was most effective, and she uses her gauntlets no matter the situation.
So if Gems aren’t vulnerable to piercing attacks, then how was Sadie able to defeat one? Clearly the monster on the island must have been particularly weak. We know that some Gems are stronger than others; Garnet having two gems might explain why she’s stronger than Amethyst or Pearl, but Rose Quartz only had one gem and was likely even more powerful.
The monster may very well have just been unlucky: a weak gem makes a weak form, which leads to its defeat by a human. It’s even possible that the monster’s invisibility was a sign of weakness rather than a special power: if the gem has to expend energy to give the form an appearance, it might not have had enough power left over to “color in” the form at all. Or, if the invisibility was a power, that could have been consuming precious energy rather than going into making the form durable. Either way the result is the same: an easy kill.
Of course there is one other time we’ve seen a Crystal Gem get broken up apparently by damage: when Sugilite was hit on the head by her own flail. It’s really hard to be sure if it was the damage that did it, though. Sugilite is very strong, and the flail only had the force of gravity behind it. It could have been the simple surprise of the impact that caused Amethyst and Garnet to lose their concentration long enough to break the fusion apart, just like Opal broke apart when she realized she hadn’t actually retrieved the Heaven Beetle.