Imitation Gemstones, either natural or man-made, are effective substitutes for stones that are too expensive or too difficult to obtain. One of the problems facing re-enactors is how to present an authentic appearance for a person of middle to high wealth on a limited budget. This is especially true regarding jewelry. Many gemstones that were popular in the Middle Ages are too expensive for the average person to obtain. There are several ways to use substitute or imitation jewelry however. Imitation gemstones were also popular in medieval times for much the same reasons (people trying to save money and portray someone wealthier than they were).
The use of colored glass to imitate gemstones dates back to ancient times. For small jewels this can be very effective. For larger gemstones glass may be too clear, and lacks the inclusions found in natural stones. Also, be cautioned that a lot of costume jewelry is made from acrylic not glass. Volcanic glass, a man-made substance from obsidian and volcanic ash, is a possible candidate for imitation aquamarine and pale sapphire.
Photos: ceramic scarabs [dead link], glass drop "emeralds"  foil-back glass rhinestones 
Most inexpensive jewelry on the market uses synthetic rather than natural gemstones. These fall into two categories: Laboratory synthetics are exact duplicates of natural minerals. The only difference is that the stone lacks the inclusions and imperfections found in nature. Synthetic imitations are also grown in the lab, but are of a different chemical composition than the natural stone, and are usually cheaper as well. When using synthetic stones one should remember that most medieval jewels were cut as cabochons. A facetted gemstone would look out of place on a Viking warrior. (Note, however, that crude faceted beads have been found in Viking treasure hordes.)
Photos: lab-grown opals  synthetic blue spinel  cubic zirconia  & 
There are a number of natural gemstones that bear a strong resemblance to the cardinal stones — diamond, emerald, ruby, and sapphire. The following is a list of natural stones (plus a few man-made ones) which may prove to be effective substitutes:
- adamant (diamond) — Rock crystal (a clear variety of quartz) is the classic substitute for diamonds. The best quality quartz came from central Europe, near the Rhine; hence the term rhinestone. Today, most rhinestone is made of glass rather than quartz. Swarovski crystal is considered to be the best. Another good possibility is clear topaz, but may not be available. Heat-treated white sapphire is available at a good price. Herkimer diamonds (another variety of quartz) make convincing looking diamonds in the rough.
Photos: diamonds in the rough   Herkimer quartz crystals  rock crystal "diamonds"  & 
- carbuncle (ruby) — Garnet is the best bet; it was one of the three stones classed as carbuncle.1 Spinel was a another type of carbuncle, but it has become rarer and more expensive than even genuine ruby. Synthetic spinel may be a good substitute however. For an economical substitute, carnelian is a good choice. The best quality is heat-treated, although it looks rather brown in comparison. For imitating balas ruby consider using rubellite (pink tourmaline). This stone can be rather expensive, but is generally more available (and affordable) than natural red spinel.
Photos: ruby cabs  garnet (rhodolite)  pink tourmaline 
- emerald (green beryl) — This is a difficult stone to imitate. Few other minerals exhibit such a vivid green color. Green tourmaline or dark green peridot are generally the best choices. Green zoisite (anyolite) is a possibility, but often has ruby inclusions, and is only available in baroque (tumbled pebble) form. For those on a tight budget, green aventurine is a good choice, although it lacks the clarity of fine emerald. Make sure to obtain high-grade aventurine if possible. Nephrite jade is another economical choice. Green dioptase and chrome diopside both have the right color, but are too soft and brittle. In ancient times "emerald" referred to any green stone, including malachite, which is fairly easy to obtain. Malachite has the vivid color of emerald but is an opaque stone.
Photos: emerald cabs  chrome tourmaline  aventurine  (tumbled) 
neprhite jade  (tumbled)  rough emerald 
- hyacinth (medieval & modern sapphire) — This is another difficult stone to imitate. By the time of the Crusades, the term "sapphire" was being used to refer to blue transparent gemstones, rather than the lapis of ancient times. A lot of medieval sapphire jewelry has proved to be nothing more than colored glass. If one wishes to use a natural gemstone substitute, the best choice, based on current prices, is iolite—also known as "water sapphire"—although this stone is often more purple in color than true sapphire. Blue tanzanite is far too expensive except perhaps in baroque form. Another possibility is blue topaz. This is not a naturally colored stone however. The blue coloration is produced by irradiation—either by x-ray or neutron bombardment—and may have a tendency to fade over time. Be sure to ask for "London blue", as most blue topaz is rather pale in comparison and perhaps better suited for imitating aquamarine.
Photos: sapphire cabs  iolite  aquamarine  blue quartz 
blue topaz (irradiated - scroll up)  (l - r) iolite, aquamarine, apatite, blue topaz 
- NOTE: 1. The meaning of "carbuncle" has changed over the years. In the modern jewelry trade it refers to a cabochon that has been hollowed out. This is often done with large almandine cabs to lighten the color.
- beryl (aquamarine) — First of all, bear in mind that most aquamarine found in nature is pale green in color NOT pale blue. Aquamarine jewelry is usually heat-treated to obtain the desired blue color. Aquamarine baroques are fairly cheap and exhibit a cloudy gray color with just a hint of turquoise or sky blue. Aquamarine gemstones are not terribly expensive, but may not be available in the desired cut. For the pale blue gem variety, kyanite approximates both the color and the crystal structure of beryl, and is readily available in bead form. For the more natural sea-green type, prehnite is a reasonable choice. Some varieties of apatite have a vivid blue-green color, but the stone is rather soft and brittle.
Photos: aquamarine (tumbled)  kyanite  sapphire  blue topaz  prehnite 
- amethyst — This gemstone is relatively inexpensive and easy to obtain. Purple fluorite approximates the color, but is very soft in comparison. Photo: 
- chryselectrum (golden topaz, chrysoberyl, etc) — Citrine is the obvious choice here. Smoky quartz is another possibilty, although it looks rather brown. Organic amber is a separate category, and is relatively cheap. Photos: Imperial topaz (scroll up)  (crystals) [dead link], tumbled citrine (heat treated)  (& natural)  citrine cabs 
- callais (turquoise) — Turquoise is still relatively inexpensive, but the price is going up. Dyed howlite is a cheap alternative (but check the cautions for lapis above). For a durable substitute consider amazonite instead. The color is just about right, but it does lack the copper inclusions.
Photos: Turquoise nuggets  Amazonstone 
- chalcedony — Fairly cheap but not always obtainable under that name. Try asking for blue-lace agate, which is nearly identical.
Photos: chalcedony  blue-lace agate  blue calcite 
- opal — Genuine opal is both expensive and fragile. The natural stone tends to dry out over time. Opal is also sensitive to temperatures changes and can crack without warning. Fortunately, there are several stones offering the play of color and iridescence for which opal is famous. Depending on the base color of the opal being imitated, one might consider moonstone, especially the "rainbow" variety. Another good possibility is spectrolite, a high grade variety of labradorite.
Photos: opal  moonstone  labradorite  spectrolite cabs 
- ultramarine (ancient sapphire/lapis lazuli) — Good quality lapis has become somewhat expensive, and a lot of the stuff on the market is of the "denim blue" variety. Sodalite is a good substitute although it lacks the pyrite inclusions found in genuine lapis. Dyed howlite is another possibility. Be warned, however, the dye job is seldom permanent.
Photos: lapis (tumbled)  sodalite 
From a 14th century statue.