The Asscher brothers, Abraham and Joseph Asscher, are credited with creating this diamond cut which goes by their name even to this day. Founder owners of the Royal Asscher Diamond Company in 1854 in Amsterdam, the Netherlands, the brothers fashioned a square, step-cut shape with cropped corners. The Asscher cut was developed in 1902, at the advent of the Art-Deco movement that is characterized by the straight-lined geometric form. Very popular till the 1920s, the shape later found expression in the emerald cut shape of diamond.

Carat The weight used in the gem trade since antiquity. The name is derived from the seed (kuara) of the African Coraltree or from the kernel (Greek -kertion) of the Carob bean. Since 1907 Europe, as well as America, has adopted the metric carat of 200mg or 0.2g. Therefore weights given for famous old diamonds often vary because local carats and not metric carats were used. The carat is subdivided into fractions (l/10ct) or decimals (1.25ct) up to two decimal places. Small diamonds are weighted in "points" = l/100cts (=0.0lcts).

Weights used in the gem trades

Optical properties

Specific Gravity

Specific gravity (also called "relative density") is the weight of a specific material compared with the weight of the same volume of water. A gem with the specific gravity of 2.6 is therefore 2.6 times as heavy as the same volume of water.

Cleavage and Fracture
Many gems can be split along certain flat planes, which the expert calls cleavage. Cleavage is related to the lattice of the crystal - the cohesive property of the atoms. Depending on the ease with which a crystal can be cleaved, one differentiates between a very perfect (euclase), a perfect (topaz) and an im¬perfect cleavage (garnet). Some gems cannot be cleaved at all (quartz). A loosening of contact twins is not cleavage but separation.

Hardness
In the case of minerals and gemstones, hardness refers first to scratch hardness, then to cutting-resistance. Before the development of optical testing methods, scratch hardness was of great importance in the determination of gemstones. Today the hardness test is only rarely applied to precious stones and then mainly by collectors. It is too imprecise for an expert and the danger of damaging the gem is too great. The advantage of the scratch test is that one can, with simple means, roughly determine the nature of a stone. It is frequently used in mineralogy.

Gemstones and their influence
Gems have intrigued men for the last 7,000 years. The first known were amethyst, rock crystal, amber, garnet, jade, jasper, coral, lapis lazuli, pearl, serpentine, emerald and turquoise. These stones were reserved for the wealthy and served as status symbols. Rulers sealed documents with jewel-encrusted seals, which were an expression of their wealth and power.

As, with a few exceptions, most gemstones are minerals, we must concern ourselves with the origin and structure of these minerals. The formation of the non-mineral gemstones (for instance amber, coral and pearl) will be dealt with in more detail when they are described.