There are five gems (four gems and an ingot technically) which I group together, as they all make blocks, armor, and tools. They are topaz, sapphire, cobalt, peridot, and amethyst. Any stat that is low is below diamond, if it is medium it is at diamond, decent, it is a little higher than diamond, and high is a good bit above diamond. Every tool can mine as high as diamond can.  They all have to be mined by diamond.

Topaz: Medium Durability, high mine speed, low damage, high armor.

Sapphire: Low Durability, High Mine Speed, High Damage, Low Armor

Cobalt: Low Durability, Low Mine Speed, High Damage, High Armor (Full Set Bonus: Water Breathing)

Peridot: Low Durability, Medium Mine Speed, High Damage, High Armor (The most powerful, about the same tier as ruby)

Amethyst: Decent Durability, Low Mine Speed, High Damage, Medium Armor