Larger diamonds are much rarer and hence the value goes up disproportionately.
For example:
1 carat  diamond total price $ 5000
2 carat diamond total price $15000
The price per carat does not increase smoothly with increasing size. Instead, there are sharp jumps around milestone carat weights, as demand is much higher for diamonds weighing just more than a milestone than for those weighing just less. As an example, a 0.95 carat diamond may have a significantly lower price per carat than a comparable 1.05 carat diamond, because of differences in demand.

Emeralds

Emeralds are part of the Beryl gem group.
Columbia and Brazil are the world’s largest suppliers of Emeralds.
Columbian Emeralds are famous for their high quality.
There are three principal Emerald mines in Columbia:
Chivor
Muzo
Coscuez

Experts can tell the source mine for each Columbian Emerald just by looking at the inclusions.
Emeralds are very rare. Most jewellery stores (especially those outside of central Sydney and Melbourne) will not carry natural Emeralds, and even if they do, the stones are usually man made.
Emeralds measure 7.5 on the Mohs Scale.

Rubies

Rubies are part of the Corundum gem family.
Sapphires also belong to this gemmological group.
Rubies measure 9 on the Mohs scale.
Burma and Africa are the world’s largest suppliers of rubies; Burma being the most famous.
The value of a ruby increases dramatically as the size of the stone increases.
As with Emeralds, few stores carry Rubies and of those that do, the stones are mostly man made.
The distinctive silk inclusions distinguish the natural stones from those that are man made.

Sapphires

Sapphires belong to the Corundum gem family.
Sapphires measure 9 on the Mohs Scale.
Thailand, Australian and Ceylon are the main sources of Sapphires.
Sapphires from each region have their own distinctive appearance.

Tanzanite
Tanzanite is distinctive blue/violet in colour. It is from the Zoisite gem group.
Tanzanite is only found in one region in Tanzania.
Major flooding of the mines a few years ago resulted in a huge increase in price.
Tanzanite is in short supply and great demand! It is very highly sought after and there is very little available on the retail market.
Tanzanite measures 6.5 on the Mohs Scale.

Peridot

Peridot is from the forsterite olivine family.
It is bright yellow-green in color.
It has a hardness of 6.5 on the Mohs scale.
Peridot is one of the few gemstones that come in only one color. The depth of green depends on how much iron is contained in the crystal structure, and varies from yellow-green to olive to brownish green.
Peridots are predominently sourced in Australia, China and Brazil.

Opal

Opal shows a variable interplay of internal colours.
It can range from colorless through white, milky blue, gray, red, yellow, green, brown and black.
The colors are causing the refraction of light through the internal structure of the opal.
Australia is the major source of opals.

Tzavorite
Tzavorite comes from Kenya and gets its name from the Tzavo National Park and the Tzavo River where it was first discovered.
Tzavorite is a variety of Garnet.
Tzavorite is green in colour; ranging from spring like pale green to deep forest green.
Tzavorite measures 7 on the Mohs scale.

Tourmaline
Tourmaline is in fact a group of several different but closely related minerals.
Tourmaline probably occurs in more shades of more colours than any other gem stone.
Tourmalines come from Sri Lanka, Maine, California, Madagascar, Africa, Russia, China and Afghanistan.
Tourmalines measure 7 on the Mohs Scale.

South Sea, Tahitian and Akoya Pearls
Pearls are cultured by inserting a bead under very precise conditions into an oyster. The oyster then reacts to the foreign body by producing layer upon layer of nacre around it. This process can be natural or farmed. Natural pearls occur when the foreign body enters by chance. Due to the chance element involved in this, natural pearls are rarely commercially farmed.
Pearls are one of the few gemstones that do not require any cutting or polishing because they have a natural finish. Pearls are also classified by shape; button, drop, baroque (irregular) and round.

Cultured Freshwater Pearls
Cultured freshwater pearls grow in mussels by a similar process to that of cultured seawater pearls in oysters. Foreign material enters a mussel and cannot be expelled. To reduce the irritation, the mollusc coats the intruder with the same secretion it uses for shell building, nacre.

Platinum
A white metal and more expensive than white gold.
Platinum is usually 90% or 95% pure; other alloys are added to increase hardness.

Mohs Scale