‘Simulant’ diamonds are not synthetic diamonds, but synthetic crystals made from other substances entirely which look like, replace or in some way simulate diamonds.
Rhinestones, paste (high-lead glass): the popular early alternative for diamond. Lead glass scratches easily, and many rhinestones have air-bubbles or other flaws in them. However, antique examples can be valuable.
Synthetic rutile (titanium dioxide), first created in the late 1940s, has an exaggerated yellowish brilliance and prismatic fire which many people find over-the-top. Like strontium titanate, created a few years later, it was too soft and easily damaged to remain popular. YAG (yttrium aluminium garnet), a more durable lab gem first created during the 1970s, is not as refractive or brilliant as diamond. It was replaced by GGG (gadolinium gallium garnet), another hard, bright synthetic crystal which unfortunately tended to turn brown after prolonged exposure to sunlight. These are not much found today.
White sapphire (corundum) has long been lab-synthesized and is considered one of the best simulated diamonds for an alternative to real diamonds. It should be cut to flatter the particular properties of sapphire, which is not quite so bright as diamond. Spinel, again, in its colorless form as a diamond simulant, is lab-created. It too looks a little ‘dead’ or ‘glassy’ compared with a diamond of the same cut. Colorless zircon (not the same as cubic zirconia), topaz and quartz can be cheaper even than a synthetic simulant and beautifully transparent. If in doubt, ask for certification from the Gemmological Institute or another reputable certifying lab.
Cubic zirconia (CZ, ‘Russian Diamond’, etc) is, in the twenty-first century, still one of the most popular alternatives to diamonds. CZ shows more fire than diamonds, though less brilliance. It varies in quality depending on the stabilizer used to produce it. Check out this site for more information about CZ and other created gems.
Moissanite (silicon carbide) closely approximates the hardness, brilliance and density of diamond. However, it is much more costly than CZ, and often exhibits needle-like inclusions along with a faint greenish tinge. Like rutile, moissanite can easily be distinguished from diamond by the fact that it refracts light twice, so everything visible inside the stone (facet lines, inclusions) appears ‘doubled’.