Ofcourse , first & very basic difference is Semi Precious Stones are natural stones, where as Imitations are man-made stones., like American diamonds
Semi precious stones as a category is so vast that it comprises almost every other stone apart from Diamond, Ruby, Emerald, Sapphire and Pearls. There are some 130 types of stones, minerals and crystals that can be considered as Semi Precious Stones.
These are natural & original gemstones. In Semi Precious stones there is a wide range of colour shades & shapes too.
In semi precious stones one can find vibrant shades of, purple, green, blue, orange, yellow, red, etc
Some examples of the Semi precious stones are, Topaz, Amethyst, Citrine, Quartz, Peridot, Garnet, Aquamarine, Iolite, Onyx.
These stones look more beautiful with shinning silver metal.