Yay - a 'rocky' topic!!

dubdub - you may wish to explore the characteristic of 'toughness' in relation to materials science - and in this case, gemmology. (See Post Edit below)

The crystals in jade are, at a microscopic level, fibrous - brought about by their elongate nature.

So, in contrast to many other crystalline materials, jade doesn't cleave readily. Consequently it can be both a dream and a nightmare to an artisan.

The lack of ready cleavage means that intricate fine detail is possible and that jade is suitable for tasks not generally considered the domain of minerals and rocks. The Maori picked up on this - jade was a favourite material for war clubs.

It's a bit of a nuisance for artisans as it is difficult to polish. Not saying it can't be done, but it is difficult - poor polishing will pit the surface rather than take it back evenly.

stales

POST EDIT: Since making this post I found the following at the South Australian Dept. of Primary Resources web site. Ties together what I wrote above and sjm's reply below.

Nephrite Jade

Jade has been used and appreciated by humans for thousands of years. It was fashioned into ornaments, tools and weapons by Neolithic people and has been used for religious tokens by Aztec, Maya and Maori cultures. The Chinese have revered jade above all other gemstones and it has a unique place in Chinese history. Jade artefacts have been found at ancient sites of Neolithic culture in Europe, Asia, and both North and Central America. Some of the oldest sites, such as one in Siberia, extend man’s use of jade back beyond 5000 BC.

The term jade is used to describe two distinct minerals — nephrite and jadeite. Both materials are tough, compact and fine grained. They may be similar in appearance, but belong to different mineralogical groups. Nephrite jade belongs to the amphibole group, and jadeite belongs to the pyroxene group.

Nephrite, the traditional Chinese jade, is a monoclinic amphibole that ranges between the end members tremolite (Ca2Mg5Si8O22(OH)2) and ferroactinolite (Ca2Fe5Si8O22(OH)2). Nephrite can be white, yellow, green and brown to black, depending on the amount of iron substituting for magnesium. Iron-poor tremolite is white, grey or green; whereas iron-rich varieties (actinolite) are darker green, grey-green, and grading though to black.

The most important property of nephrite jade is its toughness, which explains its widespread use for axe heads, knife blades, and delicate, durable carvings. Toughness in this context is the opposite of brittleness, and is a quite different property to hardness, which is simply the resistance of a material to scratching. Diamond is the hardest mineral, whereas nephrite is the toughest, rivalling modern artificial zirconium ceramics. The toughness of nephrite is due to its interlocking meshwork of fine fibres or needle-like crystals which are commonly 0.1–5.0 µm (microns) in diameter and 20–150 µm long. These fibres usually are arranged in bundles, with the toughest nephrite having bundles of very fine fibres arranged in random interlocking orientation. Coarse grain size and the presence of foliation (alignment of fibres or fibre bundles) reduces the toughness of nephrite.