The quality of the gem also determines whether it is considered precious or semi-precious. There are gemstones, such as garnets and tourmalines, that are more beautiful, rare, durable, and costly than the 'precious' gems. A small, fine quality deep green garnet called tsavorite would be more costly than a low-quality emerald; a brilliant red spinel more costly than a low or medium quality ruby. And a rare, brilliant, “neon” tourmaline from Paraiba, Brazil, will cost as much or more than a rare Burma sapphire.

There is no precise defintion of what a 'gemstone' is. It is defined largely by the rarity and value of the stone. Some well known semi-precious gemstones are:

Remember your 4C's when you buy gemstones - COLOR, CARAT, CLARITY, and CUTTING.

Color is all important in colored gems. The "best color" is complex, being a combination of hue, saturation and tone. Don't assume that the darker the color, the better the stone. That isn't true: color can be too dark, like some rubys that look more brownish or orangish red than blood red. The more bright and vivid the color, the better. In precise grading terms: clear, medium-tone, intense and saturated primary colors are the most preferred. Muted colors and colors between hues, which you might find very attractive, are usually less expensive. Look at the color in different kinds of light.

Clarity is the color and 'clearness' of the gem. Hold it up to the light. Can you see lots of black specks (inclusions). Clear transparent gemstones with no visible flaws are the most valued. There is no standardized grading system for clarity: it varies by gem variety. With colored gemstones, if the inclusion doesn't show in the face up position, it generally doesn't matter at all. (unlike diamonds which are graded upside-down at 10x magnification). Some varieties, notably emerald and red tourmaline, are very rare without inclusions of some kind so the price structure takes this into account. Pastel colored gemstones show inclusions more, so they generally detract more from the value for pale stones. In rare cases, inclusions can increase value. Special effects like the star in star sapphire and the eye in cat's-eye chrysoberyl are caused by inclusions. Inclusions can also be a birthmark, proving that a gemstone is from a particular place. So "horse-tail" inclusions in demantoid garnet make it more valuable because they prove it came from Russia.

Carat is the weight of the stone. Gemstones are sold by weight, not by size. Prices are calculated per carat. A carat is one-fifth of a gram. Some gems are denser than others so the same weight stone may be a different size! For example a one-carat emerald is a bigger than a one-carat ruby. Just like diamonds, the carat weight also affects the price: large gemstones are more rare, so the price per carat is higher. But practically, this doesn't make much of a difference with common gems like amethyst, citrine and blue topaz. It really kicks in for ruby, emerald, sapphire, alexandrite, tsavorite and demantoid garnet, Paraiba and rubellite tourmaline, spinel, and pink topaz. Anything above 100 carats is classed as a collector's stone. Far too big to be mounted in a necklace or ring, it's a stone that you keep wrapped up in cotton wool, and get out on special occasions.

Finally, the cutting. Are all the facets of the stone equal and well-cut, giving the gem life and sparkle. A gemstone with a good cut is something that may not cost more but can have a positive or negative effect on it's beauty. A well-cut faceted gemstone reflects the light back evenly across its surface area when held face up. If the stone is too deep and narrow, areas will be dark (aka extinction). If it is too shallow and wide, parts of the stone will be washed out and lifeless (aka window). The best way to judge cut is to look at similar gemstones next to each other. Dark areas in the stone are caused by light leaking out the back of the stones because the angles aren't right. Look for a stone having even brilliance. But the cut affects the pattern of light you will see.

Mong Hsu Rubies

Mogok rubies, which are renowned all over the world, are obtained from Mogok region in the northeastern part of Myanmar. Although these precious stones are being mined for hundreds of years, large, valuable gems are still discovered up to this day. In addition to rubies, Mogok bears Sapphire, Spinel, Peridot, Moonstone, Zircon, Quartz, Tourmaline, Apatite, Danburite, Diopside, Fluorite, Topaz and Aquamarine etc, which is unique in the world as different type of precious stones can be found in the same area.

Lower grade rubies also occur at Mong Hsu township, in northern Shan State which is not far from Mogok. Mong Hsu rubies are turned into attractive red stones when given heat treatment. It has become popular in the world market after Thai gem merchants have altered the raw stones into beautiful red ones by means of heat treatment. Heat treatment of gemstones was done in India and adjoining regions since 2000 B.C.

People in Mogok were able to bake rubies since 1950s. Blue hued rubies containing titanium oxide were baked to obtain blood red stones of intrinsic value. The tampered rubies were sold in Bangkok, Thailand, as gems trade was not allowed to be openly conducted in the socialist society. Gem merchants from Mogok too settled in Thailand and all sorts of precious stones from Myanmar were smuggled into the country and sold. Many Myanmar heat treatment experts and lapidary workers established their trade in Thailand.

During the 1970s, a Swiss couple was able to experiment with various apparatus to improve the heat treatment process of rubies and sapphire. They then imported inexpensive light coloured rubies from Sri Lanka and converted these into attractives gems using Thai workers. Thailand therefore was able to acquire the technology to temper the gemstones from various sources including Myanmar.

Rubies and sapphires are based on the same corundum mineral where oxygen and aluminium are composed. (3:2). If Chromium Oxide 1% is contained in white corumdum, valuable ruby is created from the heating process. If the white corundum is composed of 1% Titanium Oxide and iron Oxide, beautiful valvet coloured variant is obtained from conversion process.

The Mong Hsu ruby is composed of chromium, titanium and iron oxide. The stone has a blackish feature because it has black core in the C-Axis. The blue, brown and black cores which are composed of titanium and iron oxides, are excluded by means of heat treatment. Only the red chromium is left to reveal the world famous gemstone of Mong Hsu.

Although the heat treated stones change their colours, other important indicators such as specific gravity, refractive index, double refraction and absorption spectrum do not change. The Mong Hsu corundums are composed of aluminium and oxygen (Al2 O3)

Chemical Composition - Al2 O3
Crystal System - Trigonal
Hardness - 9
Specific Gravity - 3.99-4.01
Refractive Index - 1.764-1.772
Double Refraction - 0.008
Dispersion - 0.018

The most characteristic feature of the Mong Hsu rubies are their colour zoning with a violet to black core and a red rim.

The Mong Hsu stones mostly occur at Loi Hsaung Htauk mountain range which is about 17 miles from Mong Hsu town. Upper Palaeozoic Marble and other Upper Palaeozoic rocks such as Calcium silicate, Schist, Phyllite etc are observed at Loi Hsaung Htauk.

On 16th July 1992, the government declared the area as Mong Hsu Gemsland and allowed private enterprises to mine for rubies.