Sri Lankan Geuda, Ottu, and Lewuda Gemstones

Hey there, gem lovers! If you have ever been fascinated by how a rough, dull stone dug straight out of the mud transforms into a sparkling, multi-million dollar gemstone, you are in the exact right place today.

In the gemstone industry, Sri Lanka is legendary not just for its ready-made gems, but for a highly specialized traditional science known as Heat Treatment. Before these rough stones touch the furnace, expert miners and treaters sort them into very specific categories based on their internal colors, inclusions, and textures.

Today, we are opening up the ultimate traditional gem dictionary and breaking down all 36 raw gemstone types that expert treaters look for. Let’s dive straight in!

Part 1: The "Ottu" Family (Gemstones with Color Pockets)

"Ottu" refers to rough stones where the coloring elements are locked in specific spots, lines, or patches rather than being distributed evenly. Heat treaters use high temperatures to melt and spread this color across the whole crystal.

Rathu Ottu (Red Ottu): Colorless or very pale corundum stones that feature a sharp red or pink patch. When baked at the right temperature, that red spot spreads out, transforming the stone into a high-value Ruby or Pink Sapphire.
Pita Ottu (Outer Ottu): In these stones, the color zone sits right on the surface. Lapidaries must cut and polish them with extreme precision; if they accidentally shave off that outer layer, the entire gem turns into a worthless colorless stone.
Atul Ottu (Internal Ottu): The exact opposite of Pita Ottu. The color patch sits deep within the internal core. Because the color pocket is safely guarded inside, heat treaters love these stones as it is very easy to diffuse the color evenly.
Iri Ottu (Line Ottu): When held up to the light, you can see distinct, parallel blue lines running straight through the stone. The heat treatment process melts these lines, blending them together into a uniform, rich blue sapphire.
Url / Ural Ottu (Vein Ottu): Similar to Iri Ottu, but instead of thick lines, these contain a super fine web of blue lines that look like a spiderweb. Even though the initial color looks weak, correct heating yields stunning light blue or royal blue sapphires.
Dot Ottu (Speckled Ottu): The blue color appears as tiny dots or droplets scattered across the stone. During heating, these dots expand and bond together. The closer the dots are to one another, the deeper the final blue color will be.
Ruby Ottu (Dual-Color Ottu): An incredibly rare find where the host crystal is a red ruby, but it contains a distinct blue patch in one corner. Since it holds two opposing colors, master treaters must carefully control oxygen levels to pull out the desired shade.
Yellow Ottu (Yellow Patch Ottu): Mostly colorless stones that feature a bright yellow zone on one edge. Heating expands this yellow patch across the entire crystal, turning a dull raw stone into a premium, golden Yellow Sapphire.
Gilli Ottu (Sunken Ottu): The color patch here is buried so deeply inside that the stone looks like plain white glass from the outside. Miners have to submerge the stone in water or use specialized immersion oil under sharp light to spot the hidden blue core.
Demala Ottu (Chaotic Ottu): A traditional term used for stones with a confusing, irregular, or cross-cutting color pattern. Heating these carries a high risk of failure, but a successful burn rewards the treater with a highly valuable blue sapphire.
Part 2: The "Lewuda" Family (Translucent & Misty Stones)

Lewuda stones are naturally semi-transparent, but they carry unique greasy textures or internal mistiness that must be cleared out with fire.

Oily Lewuda: These stones look as if they have been coated in oil, giving them a natural, slippery, greasy luster. This oily texture makes the inside quite visible. They absorb heat very rapidly, turning into exceptionally bright blue sapphires.
Milky Lewuda: While these stones are fairly translucent, they hold a light, milky cloud or fog inside. During heating, this internal cloudiness dissolves completely, revealing a beautifully clean and crisp blue body color.
Young / Light Lewuda: These are geologically immature stones that lack a high concentration of coloring elements like titanium and iron. While they look very clear, they cannot achieve a dark blue shade, typically turning into light blue or cornflower blue colors.
Copper / Geuda-Lewuda Mix: These stones show a distinct brownish or reddish-brown tint inside. Due to the high iron content, managing the color during a burn is highly complex and requires expert handling to get a deep blue gem.
White Lewuda: This variety looks exactly like a piece of white frosted glass. By baking them, treaters activate the resting trace elements, clearing out the frosted look to produce either pure White Sapphires or very soft, light blue gems.
Malay Lewuda: A traditional mining term for incredibly stubborn stones with a highly dense, rigid internal structure. They completely resist taking in color under normal burning conditions, requiring specialized high-temperature furnaces to transform.

Part 3: The "Geuda" Family (The Kings of Heat Treatment)

Geuda stones are the absolute favorites of heat treaters because their heavy internal mineral formations hold the perfect ingredients to create world-class sapphires.

Diesel Geuda: Universally crowned as the king of heat treatment! Under normal light, these stones look just like diesel fuel—showing a mix of light green, yellow, or brown tints. However, when heated, the internal rutile minerals melt completely, creating the most expensive Royal Blue Sapphires on earth.
Milky Geuda: These stones have a dense, milk-white fog spreading across the entire crystal due to a heavy concentration of fine rutile needles. When baked at the perfect temperature, the milkiness completely vanishes, leaving an ultra-clear blue sapphire.
Blue Milky Geuda: A highly prized sub-type of Milky Geuda. It features the same milk-white background but already shows a soft blue glow inside. Treaters buy these instantly because the pre-existing blue color practically guarantees a 100% successful conversion.
Yellow Milky Geuda: Here, the dense milky cloud is mixed with a soft yellow hue. The heat treatment completely dissolves the cloudiness while intensifying the yellow shade, resulting in a premium, top-tier Yellow Sapphire.
Red Milky Geuda: A rare and fascinating type where a milky background is mixed with a pale pink or light red tint. When heated, the milkiness clears away, turning the stone into a highly expensive Ruby or Pink Sapphire.
Silky Geuda: These stones contain highly dense, uniform rutile needles that spread evenly inside the crystal. When light hits the stone, it displays a beautiful silk-like fabric sheen. Heating these yields exceptionally bright and lively blue sapphires.
Dun / Smoke Geuda: These stones look as if a cloud of gray or dark brown smoke is trapped right inside them. Though they look dirty at first, baking burns away the smoky inclusions, giving birth to beautifully clear blue sapphires.
Kowangu Pushparaga: These naturally carry a light yellow, orange, or brownish-yellow hue. Their unique internal chemical structure responds beautifully to heat, shifting into the highest quality, intense golden Yellow Sapphires that command massive market prices.
Deguna (Double-Zone Geuda): A unique occurrence where two distinct color zones (usually red and blue) coexist equally within the same crystal. Depending on how the treater controls the furnace temperature, this stone can be turned into either a top-tier blue sapphire or a premium ruby.
Waxy Geuda: The surface of these stones looks semi-translucent and dense, resembling a carved candle or a block of wax. Even though their external shine is low, expert treaters look past the surface and heat them to produce premium quality Yellow Sapphires.
Tobacco Geuda: These stones match the distinct brown or golden-brown color of a dried tobacco leaf. Depending on the exact balance of chromium and iron inside, heating can transform them into highly sought-after, orange-pink Padparadscha Sapphires or rich Yellow Sapphires.
Whitish-Blue Geuda: A variety highly valued among treaters, featuring a milk-white background with a very distinct light blue tint visible on the surface. This perfect natural balance of color and rutile leads to maximum-quality blue sapphires after heating.
Star / Thread Geuda: These stones contain internal silk needles that intersect perfectly at 60-degree angles. During heat treatment, the expert carefully avoids melting the needles completely. Instead, they preserve the structure so that when the stone is cut into a cabochon, light reflects off it to form a stunning 6-rayed star, creating a valuable Star Sapphire.
Vel Geuda (Veined Geuda): These stones feature a delicate, vein-like or thread-like pattern of color running through the crystal. Heat treaters highly favor them for producing vibrant golden or yellow sapphires.
Thick Milky Geuda: A variation of the milky geuda that is so dense it appears nearly opaque. The internal structure is so heavily packed that melting it requires extreme furnace temperatures and years of professional burning experience.
Young Geuda (Low-Grade Geuda): A semi-transparent stone that contains very few rutile or silk minerals. Because the raw ingredients inside are too low, attempting to pull out a rich color through heat treatment is highly unpredictable and risky.
Geuda Ottu (Combo Stone): A spectacular combination stone where the entire body shows a classic milky or silky geuda structure, but one specific spot holds a sharp, concentrated blue ottu patch. These are highly prized and intensely competed for among expert heat treaters.