There are a huge variety of natural gemstones regularly used in jewellery.
Most are minerals of some kind, the definition of a mineral being that it is solid and stable at room temperature, and that is has an ordered atomic structure (very scientific!)
There are a few exceptions to this, lapis lazuli is a rock, and there are other materials such as amber and pearls which are often used as gems.
Garnets from Mozambique, cut into baguettes
We tend to associate the names of gemstones with their colour, but in many cases the different colours of the same mineral have different names.
As an example, rubies and sapphires are technically the same, all colours of corundum except for red are referred to as sapphires. Sapphires are most known for being blue, but come in all kinds of beautiful colours.
This is mostly historical, as it wasn't possible until more recently to see the difference in the mineral structure of gems. Many of the red stones in the crown jewels which were thought to be rubies are technically spinels, a different type of gemstone.
Diamonds are generally expected to be clear, but also appear naturally in lots of different colours from pink and yellow to blues, browns and greens. Sapphires and topaz also have totally clear forms which before synthetic gems were developed were used as diamond substitutes.
The cost of stones is affected by their rarity and popularity. Traditionally stones were divided into precious and semi-precious, with diamond, ruby, sapphire and emerald being precious stones.
The most known stones and colours are the most expensive, but there are different grades. The deeper and richer the colour of an emerald, the more it will cost. A paler stone, possibly with inclusions will be less expensive. Buying a precious stone can be a complicated business, as there will be a variety of options depending on colour and clarity.
More recently this division is less important, although these stones are probably still some of the most popular and well known.
There are lots of lesser known beautiful stones, and more unusual colours of well known stones can be really lovely. If you were thinking of an emerald but your budget won't stretch to one, tourmaline, peridot and sapphire all come in gorgeous shades of green.
Gemstones also vary in terms of how tough they are, this is measured on the 'Mohs' scale, with diamond at the top. Generally harder stones are more valuable as they are more suitable for everyday wear but there are exceptions such as opals which are very fragile but can be hugely valuable depending on the colour.
Gemstones can have natural inclusions, other materials or flaws within the stone which are a different colour. In something like a perfectly clear diamond these make the stone less valuable, but with other types of stones they can create very interesting effects.
Cutting:
Stones are cut into shapes before being set into jewellery.
Keep an eye on my gemstone series for more on the different cuts and settings.