The Substitutes

One of the first steps to "real" Diamond Education is to understand how to recognize stones that are "not" Diamonds yet appear similar. There are some that are easy to detect, and a few using newer technology that can be more difficult to determine. Education is the key to protecting yourself from purchasing a misrepresented Diamond or gemstone.

1. Rhinestone is the most basic of Diamond duplicates. Made from glass "Silica - Silicon dioxide" (or plastic) with a golden foil reflective coating on the backs (these are easily identified by the "golden foil" coating, covering the back of each stone) they are very inexpensive and they sparkle!, Great for adorning clothing and costumes by the hundreds.

2. CZ's or Cubic Zirconia: (ZrO2) is the most widely used diamond substitute with many shapes and colors to choose from.  They are very pretty, have no imperfections and are easy to spot because their appearance is "too perfect". A Cubic Zirconium is not as hard as a diamond and will show wear on the facet edges with routine wear.  The cubic crystal structure of zirconia's ZrO2 is not stable in nature.  In lab-grown crystals, the addition of a few percent of calcium or yttrium oxide creates the stable gemstone.  It is harder than most natural gemstones (except diamond, ruby, and sapphires) and has a high refractive index and excellent dispersion.  CZ is about 75% heavier than diamond.  

Colorless and Colored
Transparency: crystals are transparent.
Fracture: low fracture toughness and strength
Hardness is 7.5 - 8.5 
Index of refraction is 1.474 - 2.15 (Diamond-2.417)
Lab Created

3. Zircon, (Zirconium Silicate) ZrSiO4 is NOT the same material as the artificial gem material Cubic Zirconia (or CZ). Zircon has been used as a diamond simulant both for innocent and nefarious reasons. Zircon resembles diamond in luster and fire and colorless zircons have occasionally been mistaken for diamonds by experienced jewelers. It is found in browns and greens but can be heat treated to beautiful blue and golden colors. Colorless material is produced in this way as well. As a mineral specimen, zircon is uncommon in most rock shops because attractive specimens are rare.  Besides blue, zircon can be colorless, red, green, yellow, orange, green or brown. Green zircons often have a natural radioactive component and should not be worn every day. Zircon of any color is desirable because it’s usually found in relatively clean crystals and in large sizes. Zircons well over 5 carats are common.

Color is brown, red, yellow, green, blue, black, and colorless. 
Transparency: crystals are transparent to translucent. 
Fracture is uneven 
Hardness is 7.5  
Index of refraction is 1.92 - 2.01 (Diamond-2.417)
Norway; Pakistan; Russia, Canada, New Jersey and Colorado USA. 

4. White Sapphires: (Al₂O₃ Aluminum Oxide) is an excellent diamond substitute as it is brilliant and very hard. Many larger white sapphires are illegally diffused with a blue coating and sold as blue sapphire. Large white sapphires are rare. White sapphire is always heat treated to produce, intensify or lighten color and/or improve color uniformity and appearance. Colorless. 
Transparency: crystals are transparent to translucent. 
Fracture: Very weak Conchoidal 
Hardness is 9.0 
Index of refraction is 1.762 - 1.788 (Diamond-2.417)
Burma, Thailand, India, Sri Lanka (Ceylon), Tanzania, Afghanistan, Australia, Brazil, Cambodia, Pakistan, Zimbabwe (Rhodesia), United States (Montana, North Carolina).