@Darkmeep It's enchanted with magic that is (presumably) useful in battle, which is also how she seems to spend most of her time. If the chain isn't sturdy, then it would break the first time she wore it.
Plus, it's a common trope that better enchantments require larger and higher-quality gems, as anything lesser would usually just shatter. Since he's clearly a practiced mage, and wants to be as helpful as possible, he probably applied the best enchantments that he could, which means an "enormous expensive gem."