eli5 What determines whether a gem is a sapphire, ruby, emerald, diamond, etc if those gems don’t necessarily have to be the most common thought color? For example, sapphires can be colors other than blue. What’s the difference between a red sapphire and a ruby?
The difference is that they are different kinds of minerals. A ruby is a red or pinkish version of the mineral corundum (aluminum oxide), while other colors are called sapphire. A red sapphire is the same thing as a ruby.
But an emerald is the mineral beryl with trace amounts of chromium or vanadium turning it green. Diamonds are a form of carbon arranged in a diamond cubic crystal structure.
Sapphire and ruby are both *corundum,* a crystal made of aluminum oxide, but with different colors due to trace amounts of various other chemicals. If it’s colored red, we call it ruby, otherwise we call it sapphire. There’s no difference between a red sapphire and a ruby (other than the name we give it).
Emerald is a totally different chemical compound, namely *beryl* or beryllium aluminium silicate, which can occur in its own range of colors depending on what trace elements it contains.
Diamond is yet another totally different chemical compound, made of pure carbon.
Rubies specifically are red sapphires. Both are a mineral called corundum. Emeralds and diamonds and most other gems have their own specific chemical compositions and structures. Diamonds for example are just carbon (but structured differently from graphite and coal doesn’t have much structure) emeralds are a type of crystal called a beryl. It’s a silicate made of beryllium and aluminum (Be3 Al2 (Si O3)6)
Of these gems mentioned, they are naturally clear and get their color from an impurity from another element if they have it with the exception of emeralds, which are green even when pure.
Quartz for example has all sorts of names. Rose Quartz, Herkimer Diamond, Amethyst, they are all just silicon and oxygen with an impurity giving it color. Look at the chemical compositions for the minerals that make them up to assess if they are the same thing.
All gemstones are composed of different materials, or building blocks. These blocks are what determines what kind of gemstone you’re dealing with. Gems come in ‘families’ as well. Rubies and sapphires are both in the “corundum” family. Amethyst and citrine are both examples of the “quartz” family, though there are many, many other quartzes out there. Sometimes the family only has one type of gem in it, like diamond. While all these families are determined by what building blocks the gem is made of, very, very small amounts of other building blocks might change the color or special properties of the gem, and that determines where in the family tree it is.
In general, different gems are different minerals. The one you specifically asked, funnily enough, is an exception. A ruby is just a red sapphire – both are primarily aluminum oxide, with other trace minerals in there which will mess around with the color.