Moonstone is a member of the feldspar group of minerals. It is a grayish white and sometimes yellowish mineral displaying a fascinating shimmer. Feldspars are an important group of rock forming minerals and is of common occurrence. Of the many members of this group are the varieties orthoclase, microcline and plagioclase. Only orthoclase feldspar (moonstones) are of gem significance in relation to Sri Lanka. Feldspar is a group of silicate minerals which include a series of gem varieties among which are moonstones, sunstones, labradorite and amazonite. Moonstone is the only indigenous variety typical of Sri Lanka and these are world renowned. A few pieces of microcline in the form of amazonite has been found occasionally in Sri Lanka, and its significance is purely for the record. The other members are not known to be found in this country.
Feldspar has a hardness of 6 in Mohs’s scale and its density varies between 2.56 and 2.7 in relation to the chemical composition of the different varieties. Feldspars are mostly dull in appearance and are translucent to opaque and are rarely transparent. The gem quality varieties have the property of displaying a beautiful shimmer as in moonstone or a play of colour as in labradorite. All varieties show two distinct sets of cleavage directions and are readily cleavable along these. The orthoclase variety crystallizes in the monoclinic system while the plagioclase and microcline varieties crystallize in the triclinic system. As a group feldspars could occur in the colours white, brown, red, green, and pale shades of yellow.
Moonstones are of the orthoclase group of the family of feldspars. Analysis has however revealed that its structure is in fact a combination of orthoclase and albite which is a sub variety of the plagioclase group. It is this combination that produces the shimmer and ‘blue seen ‘so favored in moonstones. Infact it actually contains albite in the orthoclase matrix. This creates an interference of light which in turn produces what is termed ‘Schillerisation’- the shimmer and the blue sheen effect.
Ideally, orthoclase and albite should be of a specific thickness, and arranged in alternating thin layers to produce the best effect. If the layers are too thick, the blue sheen effect will be lost, producing only a silky shimmer. The 'silky' sheen has been compared to the light of the moon. Hence its name moonstone. Moonstones are usually translucent although in its purest form it should be transparent and colourless. The characteristic shimmer and the blue sheen are most apparent in the orthoclase variety. The blue sheen in combination with a transparent and colourless body are rare and these make very attractive gemstones and are highly prized. In mineralogical terminology this effect is referred to as 'adularescence' and the material itself has been sometimes identified as adularia. As a common term all feldspars that display this sort of shimmer are collectively termed as adularescent feldspars. This characteristic interference of light is best observed in relation to a source of light. This effect seems to emanate from certain regions of the stone, giving the probable indication that this phenomenon is localized and not equally effective throughout. In better quality stones where the interference of light is optimum it displays a bluish tinted sheen. In order to produce the maximum effect moonstones should be cut 'en cabochon' although these are sometimes cut as flat pieces or rounded into beads of free sizes in order to get the maximum yield, in relation to the rough available.
Certain moonstones also show chatoyancy though not as effectively as other varieties that are capable of showing marked colour contrasts between the chatoyant streak and the body colour of the stone. However the skill of the lapidary could heighten this effect. The colour of moonstone is such that the line of demarcation between the eat's eye effect (streak) and the background colour of the stone does not show much contrast. Beautiful chatoyant moonstones are almost exclusively from Sri Lanka. Asteriated orthoclase is also found in Sri Lanka mostly from among the colourless varieties. However yellow colours are also found though rarely and in these the asteriated effect is somewhat of a sensation. Moonstones tend to show a characteristic yellowish tinge in transmitted light but this colour influence is fortunately not observed in reflected light, for if observed this could be detrimental to the soft and pleasing appearance of the stone. In Sri Lanka moonstones have been found both as water-worn pebbles and as 'in situ' deposits. The best known 'in-situ' locality for high quality moonstones is Meetiyagoda, while Kundasale and Balangoda are other noteworthy localities. Moonstones have also been located at Moon Plains. Another' in situ' occurrence is located at Imbulpe east of Ratnapura, where the stones display a smoky appearance with a flash of blue in them (Harder, 1992).
Sri Lankan moonstones have recorded densities between 2.56 and 2.58 the refractive indices being 1.525 and 1.520 having a double refraction of 0.005. The hardness is 6 in Mohs's scale. Orthoclase is essentially a potash feldspar (potassium aluminium silicate) conforming to the formula KAI Si3O8 Lankan material very often has stress cracks in the form of ladder like structures. These are often distinguishing features. The occurrence of amazonite (microcline feldspar) as mentioned earlier is more a rarity. A few samples have been picked up from time to time in and around Passara Lunugala regions. As observed, these are of a greenish blue colour, more inclined to turquoise blue with brownish streaks giving the appearance of a stain caused probably by oxidation of inclusions. This is by no means an attractive gemstone but could be polished as beads, should these be plentifully available. Amazonite is generally opaque and is of the same composition chemically as that of orthoclase being a potassium aluminium silicate. It has a slightly higher hardness than moonstone being 6.5 in Mohs's scale. The density varies between 2.56 and 2.58 and the refractive indices are 1.522 and 1.530 having a double refraction of 0.008.