Garnets are a group of minerals, the varieties of which have complex chemical compositions. These are of an isomorphous series, the constituent elements of which are capable of replacing each other to a remarkable degree. The chemistry of this group of minerals is quite interesting to the gemmologists and mineralogists alike in that elements like calcium, magnesium and iron (ferric), manganese and chromic oxide could replace each other in certain proportions without creating a change in the crystal form. This isomorphous replacement as it is called has caused the intermixture of chemical compositions and created distinct different varieties and colours. Nevertheless these varieties are not compartmentalized or water tight. There is gradual merging of elements from one into the other in certain ratios, which factor could also create intermediate varieties. In other words the composition of one gradually merges into the other. On a chemical basis garnets are divided into different varieties and the divisions are not sharply demarcated. As early as 1964 Anderson has noted that the intermixture of elements are mainly confined to certain varieties which he has classified under two distinct divisions, the pyralspite and ugrandite series. In the pyralspite series are the varieties pyrope, almandine and spessartite and in the ugrandite series are uvarovite, grossular and andradite varieties.
Accordingly, for gemmological purposes garnets are discussed under these main varieties:-
- Pyrope - Magnesium aluminium garnet
- Almandine - Iron aluminium garnet
- Spessartite - Manganese aluminium garnet
- Uvarovite - Calcium chromium garnet
- Grossular - Calcium aluminum garnet
- Andradite - Calcium iron garnet
Of these varieties andradite and uvarovite have not been found to occur in Sri Lanka and also the beautiful green grossular coloured by chromic oxide, marketed as tsavorite.
All varieties of garnet crystallize in the cubic system (isotropic). Garnets are of common occurrence. The superior gem quality varieties are beautiful and attractive enough to be set in very costly jewellery. Garnet is a commonly associated mineral constituent of the khondalite group of rocks of this country and as such it is quite widespread. In fact, it is an essential and more common alternative constituent to corundum in the rock types of the khondalite group. Garnets are mostly obtained from among the alluvial gravels and also from the weathered 'in situ' occurrences. As in other gem minerals garnets too occur in varying degrees of transparency, the fully transparent ones with good colour being the most beautiful. They could also display asterism in the form of a four rayed star. Even chatoyancy has been noted from among the Sri Lankan material as has been established by some brownish red cat's eyes brought to the State Gem Corporation sometime ago for export. Their examination revealed these to be garnets with microscopic inclusions which produced chatoyancy. Their most convincing diagnostic feature was the typical absorption spectrum, and the stones were singly refractive. The stones were pyrope - almandine garnets. This is probably the first such instance to be recorded. All chatoyant and asteriated garnets apparently have been of this variety.
The value of a garnet as a gemstone depends on the depth and intensity of colour, its lustre, clarity and transparency and its flawlessness. Different varieties of garnets have different properties and these will be discussed separately. Garnets are idiochromatic and as such the different colours are caused by elements such as iron, manganese, chromium, calcium, and magnesium which are essential constituent elements in the composition of the respective varieties. As a rule uniformity of colour is observed in all garnets. Red and its diverse shades are the commonest colours in which the varieties, pyrope and almandine occur. These colours are very often tinged with brown, yellow or violet. The red could sometimes be so intense as to appear black and opaque. However the smaller pieces show the colour very distinctly. The beautiful purplish tinted garnets which are fairly abundant in the Matale-Elahera regions are really an intermediate variety between pyrope and almandine. The term pyrandine has been suggested by Anderson (1974) for this intermediate type, but this term has not been favourably accepted. The term rhodolite seems to be the more acceptable term probably because of its rhododendron-red colour. Further, it sounds better as a trade name. This intermediate type of garnet is mostly confined to the Elahera regions. Here the colours are extremely fine, the stones clear and transparent and what is more are found in reasonably large sizes. The superior quality of this variety from this region is so renowned that these are sometimes identified as 'Elahera garnets' in order to make the variety more specific. Generally, the pale yellow and yellowish colours, and the orange and deep orange colours are of the varieties hessonite and spessartite respectively.