SYNTHETIC SILICATE GEMS VERSUS SIMULANTS. 

Simulants are commonly and mistakenly sold as synthetic or ‘lab’ stones of a certain mineral species.  A SYNTHETIC gem (or LAB created gem) should have the same chemical, optical, and physical properties of the natural gem.  A SIMULANT merely resembles the natural gem.  See the FTC Guide to the Jewelry Industry.

Buyers and Sellers should be especially careful of any listing of a 'synthetic' or ‘lab’ silicate. These include the following -- followed by their common simulants.

(1)  Lab Tanzanite (usually violet glass, lab corundum, lab spinel, CZ, YAG)
(2)  Lab Peridot (usually green glass, lab spinel)
(3)  Lab Emerald (CZ, glass, YAG, tourmaline)
(4)  Lab Aquamarine (lab spinel)
(5)  Lab Topaz (usually lab spinel)
(6)  Lab Tourmaline (usually lab spinel)
(7)  Lab Kyanite (lab quartz)
(8)  Lab Andesine Feldspar (CZ)
(9)  Lab Red Garnet  (usually red CZ, glass, lab corundum)
(10) Lab Chrome Diopside (green CZ, glass)

Silicates are expensive to synthesize.   True synthetic versions of (3) and (4) are available but expensive, while the author knows of no source for the others.   Peridot is 90% forsterite and 10% fayalite and synthetic forsterite is sold -- but is usually violet -- not green.   While there is nothing wrong with the simulants listed above, buyers and sellers should know what they are buying. 
Different simulants have different values, so the simulant material should be stated – such as  (a) 'YAG, simulated tanzanite' or (b) 'Glass, simulated tanzanite'.

Buyers and sellers should also be careful of:

(11) Lab Alexandrite (often glass, color change lab corundum)
(12) Lab Diamonds (CZ).

SYNTHETIC PROCESS

The synthetic process should ideally be listed – because stones produced by different processes have very different values.  There are seven common synthetic process -– listed below followed by common examples.   See Gems Made by Man by Nassau

MELT

(1)  Flame fusion  (lab corundum, lab spinel, YAG)
(2)  Czochralski pulling  (lab corundum, lab alexandrite)
(3)  Floating zone (lab alexandrite)
(4)  Skull Melt (CZ only)

SOLUTION

(5)  Hydrothermal (emeralds, corundum, and lab quartz)
(6)  Flux  (corundum, spinel, emeralds, alexandrite)
(7) HIGH TEMPERATURE HIGH PRESSURE  (Diamonds)

Methods (1) and (4) are inexpensive for the stones listed -- sometimes less than $1/carat faceted.  Hydrothermal quartz and Czochralski corundum are also inexpensive. 
Flame fusion is more expensive for exotic stones like GGG and rutile.  Methods (3) and (6) can be expensive:  $120-$180 per carat faceted for corundum and alexandrite.  Hydrothermal emeralds and aquamarines and Czochralski alexandrite might be $25-$35 per carat.  Synthetic diamonds can cost way over $1000 per carat.  Molded Glass is worth only a few cents per carat.

Corundum is the mineral species for rubies and sapphires.  Alexandrite is a variety of chrysoberyl – see GIA Gem Reference Guide. 
 

Graduate Gemologist, Ph.D.