Refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and brilliance are used to describe gems. Pleochroism or double refraction could be present. They might glow and have a recognisable absorption spectrum. Included materials or faults in a stone are known as inclusions.
Some are beryl, chrysoberyl, corundum, diamond, feldspar, garnet, jade, lazurite, olivine, opal, quartz, spinel, topaz, tourmaline, turquoise, and zircon.
The words “diamond, ruby, emerald and sapphire” instantly elicit feelings of opulence. These gemstones are considered the “big four” of the jewellery world. They are classified as precious gemstones while every other rock, mineral or organic matter (like pearls) are semi-precious gemstones.
Gemstones have a higher density than air, therefore when light enters the gemstone it will slow down and change direction, resulting in refraction. The ratio between the speed of light in the air and inside of an object is called the refractive index.
From ancient times and across all cultures, gemstones have been prized, not only for their beauty, but also for their special powers to protect, heal, guide, and enlighten. As far back as the Stone Age, crystal amulets were worn for protection against harm, as well as for decoration.