A look at what is currently available on eBay
How can you tell the difference between glass and a real crystal?
My kids bought a rose quartz crystal necklace and a large clear/white crystal from a specialty shop. Their friends are insisting the crystals aren't "real." I've heard some shops sell glass under the guise of being real crystals, but how do you tell?
One way to tell would be through how it is faceted. Minerals have defined crystalline structures that can only be cut into a gem/"crystal" form along certain fracture planes. Obviously, you can't really look at that, and neither can I. There are very few transparent minerals similar to glass enough to be mistaken (especially if you know how they fracture). Even fewer of these minerals are common enough to encounter in a normal crystal shop - as minerals that are being swapped for. To compare the possible glass against possible crystal, you'd need a sample of each, to see which one scratched (Mohs hardness) than the other.
Basically, if you aren't familiar with identifying real minerals and their crystals, and all that you have is one crystal of something, there is very little that you can do. The only really possible way to try would be to take the Mohs scratch test to your crystals. Most of these possibly transparent crystals are softer than glass, and would scratch - but then you'd have a scratched crystal.
I would recommend accepting the 99.9% chance that is is just glass, and enjoy it anyway.
Items recently purchased from this site
Natural white Crystal Yellow Jambhala CHINA Plutus GOD figure of Buddha, Daum Crystal TULIPE WHITE VASE 03375-3 New In Box MINT, FRANCK MULLER 18K WHITE GOLD DIAMOND SAPPHIRE CRYSTAL LADIES WATCH 3.68CTS, UNBELIEVABLE FRENCH EMPIRE CRYSTAL CHANDELIER WITH BRIGHT RED & WHITE CRYSTALS!!, Daum Crystal WHITE RHINOCEROS 03400 New In Box MINT, Black Leaf Style Chandelier with white Crystal, MYST DE CARTIER White Gold Diamond Rock Crystal Dome Ring, Daum Crystal PEONY JEWELS WHITE VASE 03935 New In Box MINT, Daum Crystal VINTAGE WHITE EGRETS VASE 03891-1 New In Box MINT, Rolex Day Date 18k white gold diamonds sapphire crystal ref 18039, Daum Crystal VIS DE FORMEAMP BLACK & WHITE 03246-1 New In Box MINT, Daum Crystal ELEPHANT WHITE 02568-2 New In Box MINT, Creamy Pearl White Leather Crystal King Bed Bedroom Set Master Bedroom Furniture, Unique 18K White Gold Diamond Ruby and Crystal Brooch, Daum Crystal CHEETAH WHITE 02677-2 New In Box MINT, JUDITH RIPKA 18k White Gold, Crystal & Diamond Earrings, Natural white Crystal CHINA Goddess Of Mercy Kwan yin statue Bodhisattva, ROLEX 116400 - Milgauss -Clear Crystal - White Dial - 40mm - Stainless Steel, Crystal White Lion Statue - Marble Statue, Daum Crystal ROSES POWDERED WHITE VASE 03547-3 New In Box MINT, BNIB LADIES STEPHEN WEBSTER 18K WHITE GOLD TURQUOISE CRYSTAL HAZE RING, LALIQUE CRYSTAL 17" WHITE GANSTER SHARK 11 POUNDS STAR OF JAWS BRAND NEW IN BOX, Daum Crystal VINTAGE WHITE LIONS VASE 03866-1 New In Box MINT, BOGINNI&CO.WHITE CRYSTAL CORSET BRIDAL ROSE 3 METRE WIDE PETTICOAT WEDDING DRESS, Daum Crystal TULIPE WHITE POWDERED VASE 03574-4 New In Box MINT, Zuki Ermine Applique White Fur Ladies Coat Crystal Buttons Gorgeous Mint FOXY!, Fox Coat - Crystal Fox Coat White Fox Trim, 8.0" White Jade Carved Crystal Alien Pandora Dragon Avatar Toruk Skull, Vintage Natural Opal Crystal Diamond Cocktail Ring Solid 14k White Gold Platinum, AMAZING VHERNIER 18K WHITE GOLD DIAMOND ROCK CRYSTAL SHELL EARRINGS 0.40CTS , JUDITH LEIBER Black White Crystal Golden Monkey Minaudiere Clutch Handbag, Natural white Crystal Chinese God Maitreya Buddha fengshui Gathered wealth, Daum Crystal PEONY POWDERED WHITE VASE 05115-1 New In Box MINT, GORGEOUS WHITE and BLACK MINK FUR Trench Coat Black Leather Detail Crystal Belt, Daum Crystal BLACK AND WHITE JOCKEY 02561-1 New In Box MINT, CHANEL Lambskin Swarovski Crystal Charm Hobo White , ESTATE 14K WHITE GOLD FINE 10CT CRYSTAL QUALITY OPAL & ROUND DIAMOND RING, quartz crystal white from brazil 12 kg , 18K WHITE GOLD 84.94CT CLEAN QUARTZ CRYSTAL EARRINGS NATURAL RUTILE INCLUSIONS, NEW: White Fox Fur Jacket - Crystal Fox Trim, Antique Aquamarine Diamond Rock Crystal Filigree Estate Necklace 14K White Gold , Daum Crystal WHITE BENGAL TIGER 03448-1 New In Box MINT, Fox Jacket - Crystal Fox Jacket White Fox Trim, SAINT LOUIS THISTLE CRYSTAL WHITE WINE 6 GLASSES SET #1, SAINT LOUIS THISTLE CRYSTAL WHITE WINE 6 GLASSES SET #2, Clear Crystal & Diamond White 18kt.Solid Gold Beautiful, Crystal Marble Stone Tile Floor Super White 24x24 32x32 40x40 $5.49 sqft , Daniel Lotton, Crystal Clear, White Flowers Bowl Mint, $8K NEW CHANEL WHITE TWEED LACE CRYSTAL BUTTON SUIT DRESS JACKET SKIRT BLOUSE 38, White High Hat Full Lead Crystal Chandelier Pleated Shades Hand Made New Classic, Delicate White Scolecite Natural Crystal Specimen, Crystal Glass Stone Tile Porcelain Marble Floor White, Daum Crystal ELEPHANT WHITE 03239-3 New In Box MINT, UNIQUE 34.96CT CLEAR INCLUDED QUARTZ CRYSTAL 18K WHITE GOLD WIRE EARRINGS, DIAMOND & CRYSTAL STARFISH IN 18K WHITE GOLD, Longines 14k White Gold With Original Diamond Dial And Sapphire Crystal, Beautiful 42lb. White Calcite Geode Crystal Rock, JUDITH LEIBER Pink White Floral Crystal Minaudiere Chain Strap Clutch Handbag, JUDITH LEIBER Black White Crystal Spotted Pug Minaudiere Chain Strap Handbag, JUDITH LEIBER Black White Gray Chevron Crystal Minaudiere Chain Clutch Handbag, JUDITH LEIBER Black White Crystal Sleeping Cat Minaudiere Clutch Handbag, FROSTED QUARTZ CRYSTAL LEAVES 14K WHITE GOLD LILY PIN DIAMOND SAPPHIRE ACCENTS, BRAND NEW IN BOX CERT SWAROVSKI CRYSTAL DISNEY SNOW WHITE 7 DWARFS SET RETIRED , ~c1920 Spectacular French White Roses Crystal Prisms & Murano Drops Sconces OLD~, CHANEL BLACK WHITE PEARL CC CRYSTAL BAGUETTE ANGELS NECKLACE GOLD NWT CHAIN 09A, RARE Giant Clear Quartz White Multiple Layers Phantom Crystal Point, White Jade & Labradorite Carved Crystal Polar Bear, Sculpture, amazing flash, Auth Chanel Crystal CC Logo White Pearl Silver Necklace! Eyes catch U!, ANTIQUE OLD MINE DIAMOND 14KT WHITE GOLD CAMPHOR CRYSTAL ART DECO PENDANT #2180, CHANEL $3995 BLACK & WHITE CRYSTAL EMBELLISHED SWEATER / JACKET 12/44 (51634), JUDITH LEIBER White Floral Crystal Coin Box Minaudiere Chain Clutch Handbag, Exquisite 1.5ct Harlequin Black Crystal Opal .9ct Diamond PENDANT 14k White Gold, JUDITH LEIBER Gold Tone White Blue Crystal Minaudiere Bow Evening Clutch Handbag, BOGINNI&CO WHITE AUSTRIAN CRYSTAL CORSET & ROSE DUCHESS SATIN WEDDING GOWN DRESS,