There are many names for different gemstones and minerals. Often these names are misleading as to what type of gems/ minerals they are representing, especially when they are synthetic and are referred to as nautral stones.  Here are some misleading names and the exact minerals/gems or synthetics that they actually are.  The misleading names I'll call imposters.

Diamond impostors:
German, Herkimer and Bohemian 'Diamonds' are Quartz stones.

Moissanite (silicon carbide) (lab grown, rare in nature)

Strontium Titanate (lab grown)

Cubic Zirconia is Zirconium powder (lab grown)

Rutile (lab grown)

Emerald imposters: 
African 'emerald' is green flourite,
Bohemian 'emerald' is green fluorspar
Broghton 'emerald' is green glass
Gilson & Chatham 'emerald' are synthetic emeralds
Cape 'emerald' is Prehinite Emeraldine is dyed green chalcedony
Oriental 'emerald' is green sapphire
Evening 'emerald' is peridot
Spanish emerald is glass.

Ruby imposters: 
Adelaide 'ruby'
Australian 'ruby'
Bohemian 'ruby'
California 'ruby'
Cape 'ruby'
Montana 'ruby' - all of the above are garnets
San Diego 'ruby' and Siberian 'ruby' are red tourmalines.

Star Rubies of a large size and very red is usually synthetic corundum

Sapphire imposters:
Brazilian 'sapphire' is blue tourmaline
Burma 'sapphire' is synthetic sapphire
Hope 'sapphire' is synthetic sapphire.

Star Sapphires in a very large & deep blue color are synthetic corundum

Jade imposters: 
African 'jade' is garnet
Colorado 'jade' is feldspar
Honan 'jade'  and Manchurian 'jade' are soapstone
Indian 'jade' is aventurine
Swiss 'jade' is jasper.

Pearl imposters:
Roman and Swarovski 'pearls' are glass beads
Atlas 'pearls' are calcite
Delta 'pearls' synthetic
Mallorca 'pearls' are glass beads with pearlescent coating

Opal imposters:
Ceylon 'opal' is moonstone
Blue 'opal' is lazulite

Lapis imposters:
Swiss 'lapis' is chalcedony or dyed jasper (quartz family). 
German 'lapis' is dyed jasper
Canadian 'lapis' is sodalite.

Turquoise imposters: 
African 'turquoise' and yellow 'turquoise' are jasper
Chinese 'turquoise' is calcite or soapstone
Sacred 'turquoise' is smithsonite
Utah 'turquoise' is variscite
Vienna 'turquoise' is synthetic or reconstituted turquoise.

Alexandrite imposters:

Synthetic Alexandrites usually change from purple to red/green and are created synthetic Corundum (synthetic sapphire).  Natural alexandrites emit color changes closer to blueish/red/green tones and the generic name is chrysoberyl.  Large natural Alexandrite is very rare. 

Kunzite imposters:

Kunzite is often mistaken for bluish-pink amethyst, which is quartz

Aquamarines:

Produced and synthesised by hydrothermal growth. 

Information source: About.com/Misleading Gemstone Names by Cary Wickell

Information source:  Fifth Edition GEMS, their sources, Descriptions & Identification by R. Webster

Janet Deleuse

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