Amethyst shards for combat-related enchantments

You should be able to choose whether you want combat or non combat related enchantments depending on whether you use amethyst or lapis. If you use amethyst, you're more likely to obtain enchantments like sharpness, cleaving, and protection. If you use lapis, you're more likely to obtain enchantments like unbreaking, fortune, and infinity. This would give amethyst a combat-related use.