Beryl group of gemstones

by Manisha G

gemstones that belong to beryl groupChemical Composition : Silicate of beryllium and aluminium.

Crystal Characteristics : Hexagonal system with full hexagonal symmetry. Crystals usually long, prismatic, terminated by a flat face called the basal pinacoid. In other beryls and aquamarines pyramid faces are often present in addition to the basal pinacoid. Emerald crystals are commonly flawed, aquamarine and beryl occurs in larger crystals than emerald and are often free from flaws.

Varieties : Intense to pale velvet-green to grass – green-emeralds; pale green (green beryl); pale blue to greenish blue – aquamarine; yellow-golden beryl (heliodor); pink – rose pink beryl (morganite); colourless – (goshenite); magenta, brown and a variety of clear green beryls distinct from emeralds are also known.

Physical Properties show a fairly considerable range of values in­dicating some degree of isomorphous replacement.

Cleavage : Very indistinct – parallel to the basal plane.

Hardness : 7 1/2. Slightly variable. Emerald is brittle and care must be taken in cutting and setting.

Specific Gravity : 2.67 to 2.80. All beryls will float in bromoform (S.G. 2.88). A test which is sometime useful in rapid testing of loose stones.

Luster : Vitreous to resinous.

Refraction : Double, uniaxial, negative sign, R.I.-1.56-1.57 to 1.59-1.60 (D.R. 0.005 to 0.009)

Dichroism : Distinct to faint. May be difficult to detect in a pale stone. Twin colours in a good coloured emerald – yellowish – green and dark bluish-green. In aquamarine – blue and green or colourless.

Deep-blue aquamarines often possess exceptionally strong dochroism. Deep coloured aquamarines are rare and need to be cut carefully to exclude greenish dichroism.

EMERALD

Flawless stones are extremely rare and most specimens are marked by flaws and inclusions which greatly reduce their transparency. Stones clouded by fissures are termed “mossy”. Distribution of colour is not always uniform, and may be arranged irregularly or in layers parallel to the basal plane. Green fluorspar is practically the only natural mineral having an emerald green colour, though fine translucent chalcedony, green jadeite, tsavorite garnet and certain heat-treated green tourmalines approach it very closely. Demantoid garnet has a yellow and more intense green and is usually a far livelier stone with its high R. I., luster and dispersion. Clever imitations of paste and composite stone (doublets) are often encountered.

Emeralds owe their fine colour to the element chromium which has to be present in detectable quantities (0.03% or more) to classify the stone as emerald. All other green varieties of beryl which owe their colour to other elements have to be classified as green beryls.

Beryls of fine grass-green (emerald-like) colour, have been found in various places. The colouring agent in these cases is Vanadium with some iron which may or may not be present. These beryls have been found in large quantities and often found as, near flaw­less crystals having constants similar to emerald. Green beryls which owe their colour to iron can be heat treated in controlled conditions to give a permanent colour change to blue (Aquamarine) and this process is commercially carried out.

AQUAMARINE

Sometimes found in quite large transparent crystals comparatively free from flaws. The colour is probably due to small amount of iron.

Many blue-green aquamarines are subjected to controlled heat treatment which greatly improves the blue colour. The colour change is permanent.

A very common imitation of aquamarine is made of pale blue synthetic spinel and blue topaz. This can be detected by their much higher R.I. and S.G.

PINK BERYL (MORGANITE)

The colour is probably due to traces of manganese. The rare alkali metal caesium is also present and is responsible for the higher physical constants. These elements probably replace part of the beryllium in the crystal lattice.

YELLOW BERYL (HELIODOR)

The colour is due to iron and varies from a pale greenish yellow to an intense golden yellow. In the latter quality, it is a most attractive stone.

MAXIXE TYPE BERYL

Early in the 1970s beryls of a striking cobalt-blue colour appeared as attractive cut stones on the market in New York and London, and fetched high prices. The properties of these beryls were reminiscent of blue beryl found in the Maxixe mine in Minas Gerias some forty years previously, and like these attractive stones the new ones showed a marked tendency to fade in bright sunlight. These ‘Maxixe’ type beryl should be treated with caution as being liable to fade after prolonged exposure to light.

Raspberry red colour material called bixbite is the red colour beryl found in St. Thomas Range, Utha, USA.

CUT

Emeralds – Usually step or trap-cut which is also known as “Emerald cut”; but also cut cabochon and brilliant.

Aquamarine – Step, mixed or brilliant. In large stones the cut is often modified by many additional facets.

Detection : All beryls may be detected by checking R.l. and S.G. since no similar stones overlap the range of these stones.

In addition to synthetic spinel, blue topaz may closely resemble aquamarine. Their constants are quite dissimilar and identification should not be difficult.

Pink Beryl, or morganite, may easily be confused with pink topaz, kunzite, pink spinel, tourmaline. All may be separated quite easily by comparing their R.l. and S.G.

Yellow beryl, or heliodor, may be similarly mistaken for other yellow stones and just as readily identified. Citrine is sufficiently lower in R.l. to dispel and doubt between the two stones. Yellow orthoclase feldspar is close to yellow beryl in colour. Again the R.I’s are lower than those for beryl.

Leave a Comment

Previous post:

Next post: