Turquoise is a semi precious bluish stone which goes greenish with oxidisation (exposure to air) and age.
Most Turquoise that I know anything about comes from Afghanistan.
When I first started selling Turquoise jewellery I bought from Nepal where the jewellery was set. The stones came from Jaipur in India where they are cut,.or directly from Afganistan. After the troubles began in Afganistan, it became harder to come by and now the supply has dried up. So I bought it from Hong Kong where massive factories imported stones from around the world and superceded Jaipur by the speed of their machinery. It was sad as Jaipur had been the world's premier stone cutting centres for over 3000! years. As an aside Jaipur is a fascinating place to visit. The history of the jewellery trade can be evidenced by some of the shops and workshops which have passed down the generations, with little change except for electricity.
Therefore due to the increasing rarity first they started dying white howlite blue to make it look like Turquoise and even improved(?) on it: the colour of brighter blue which is more popular with customers. Many wanted real Turquoise but bright blue Turquoise, which if it exists at all finally goes green.
Then a few very clever and hardworking people (in concert or separately) finally perfected artificial resin (sorry I can't remember the chemical names, there are a few different ones), which has eluded many for a long time.This is a lot easier to mold into jewellery (if you know what you are doing and have the correct equipment) but at first was very expensive, as all new things are, due to the experimentation costs involved.
Over the last few years many improvements have been made to make the blue resin look like Turquoise, so that as with others stones it can be very hard to tell the difference. One tell tale sign of resin is leakage around the edge of the jewellery, which hasn't been cleaned up properly, also if the jewellery is open at the back you can sometimes see more mistakes which show it is resin.
Some blue resin and Sterling Silver earrings:
I think that Howlite is a smoother stone (or more easily tumbled, cut and polished) than Turquoise with sharper dark lines (these lines can of course be coloured!). Howlite I believe is in nature white.
Reconstituted Turquoise is crumbled Turquoise (and I believe, other things) heated and mixed with a gluey resin. The end result can be manipulated into natural shapes which have the close appearance of Turquoise.
But just because you are getting blue resin or blue howlite jewellery this doesn't necessarily mean it will be cheaper than Turquoise. It's often the work involved, other materials used and also the buying power of the manufacturer of the jewellery which will determine price (among other things).
For more information, corrections or any insights that I have overlooked please feel free to email me on this fascinating subject.