Colour and General Appearance.
Colour is one of the most variable properties of greenstone, and most of the classes founded on petrographic grounds are found to include specimens ranging from greyish and even whitish tints to
various shades and depths of green. Nevertheless, colour is often an important indication of the petrographic class to which a rock belongs, and is especially useful in distinguishing serpentines from nephrites.
In general the colour appears to be more uniform in the serpentines as a class, yellowish sea-green and bluish-green tints being especially characteristic, although occasionally observed also in the nephritic greenstones. Serpentines, too, seem on the whole to be more translucent than the nephritic rocks, and, furthermore, often have a characteristic finely-banded appearance on polished surfaces, not unlike the grain of polished wood. On the other hand, most of the opaque greyish-green specimens are tremolite-rocks and semi-nephrites, while deep grass-green, subtranslucent, homogeneous types are usually true nephrites. Among the serpentines many of the tangiwais have a fairly deep green tint; but these can usually be distinguished from nephrites by their greater translucency and mottled appearance. The above generalisations are subject to many exceptions, but nevertheless serve as useful guides in hand-specimen determination.
Finlayson (1909, pp. 369 to 371) has discussed fully the causes of variation in colour of nephrites. He concludes that the depth of green is proportional to the amount of ferrous iron replacing magnesium in the constituent tremolite or actinolite. It may here be noted that the brilliant emerald-green spots observed in very rare specimens of nephrite (e.g., 1804) have been found to be due to the presence of aggregated crystals of chrome-diopside, while in one or two specimens (e.g., 1839) patches of green talc produce a somewhat similar appearance.