Cuts:

Brillant Cut	Emerald Cut	Ladder Cut	Cabochon Cut	Cabochon Cut
Pear Cut	Marquise Cut	Oval Cut	Princess Cut	Baguette

Mohs’ Hardness: The Viennese mineralogist Friedrich Mohs (1773 – 1839) chose 10 minerals, which scratched the posterior minerals in the scale, but were scratched by the previous minerals. His scale is now universally accepted scale for minerals’ hardness:

Mohs’ hardness	Mineral used for comparison	Simple hardness tester	Generic hardness
10	Diamond		hard
9	Corundum (Sapphire, Ruby)		hard
8	Topaz		hard
7	Quartz	scratches window glass	hard
6	Orthoclase	can be scratched with steel file	medium hard
5	Apatite	can be scratched with knife	medium hard
4	Fluorite	easily scratched with knife	medium hard
3	Calcite	can be scratched with copper coin	medium hard
2	Gypsum	can be scratched with fingernail	soft
1	Talc	can be easily scratched with fingernail	soft

Cleavage: cleavage is when the gemstone can be cut parallel to its flat surface and depends on the structure of its crystals. It can be perfect (diamond, Labradorite, moonstone), imperfect (peridot, zircon) or it can be absent (amethyst, chalcedony, citrine). The jeweller has to be very careful with the cleavage during cut and setting of the gemstone, as it can break. The more perfect the cleavage of a gemstone, the more difficult is to cut it.

Fracture: break due to a hit.

Density: also known as ‘specific weight’, it ranges from 1 to 7 in precious and semiprecious stones. Its formula is:

Density = Weight of the gemstone / Volume of the gemstone

Pleochroism: in some transparent gemstones, the different aligning and structure of the crystals can originate different degrees of light reflection and therefore, different colour intensities or even different colours inside the same gemstone. Very pleochroic gemstones are sapphire, ruby or emerald.

Asterism: depending on the aligning of the crystals and of the inclusions of the gemstone, light can be reflected in shape of a star of four, six or even 12 spikes. To fully appreciate this phenomenon the stone needs to be cut in form of a cabochon.

Cat’s eye effect: if the crystals or the inclusions of the gemstone are aligned parallel within the whole stone, light can be reflected as a white line on the gemstone, which reminds the eye of a cat. To fully appreciate this phenomenon the stone needs to be cut in form of a cabochon.

Fluorescence: It is the luminescence of the gemstone after having been exposed to ultraviolet light. Its reason are minerals inherent in the stone. In some cases, this method is essential in determining the type of gemstone or its authenticity.