Learn essential techniques for assessing rough opal potential. Identify colour bars, understand potch lines, and spot inclusions to make informed buying decisions for your next cutting project.

A Cutter’s Guide: Assessing Potential in Rough Opal Stones

So, you’ve got your hands on a piece of rough opal, or you’re looking to buy some. The excitement is palpable! Hidden within that seemingly ordinary rock could be a galaxy of colour, a breath-taking pattern, or the heart of a stunning piece of jewellery. But how do you assess rough opal potential and gauge what treasures might lie within? This guide will walk you through the key indicators to look for, helping you “read” rough opal like a seasoned pro.

First Impressions: The Initial Inspection of Rough Opal

Before diving deep, give the rough opal a good initial once-over.

• Clean the Stone: If it’s covered in dirt or host rock, gently clean it with water and a soft brush. This helps reveal the true surface.
• Lighting is Key: Use a good, strong light source. Natural daylight is often best, but a reliable lamp can work wonders. Observe the stone from all angles.
• Wet and Dry Examination: Some features are more visible when the opal is wet, while others show better when dry. Don’t be afraid to get it wet (most opals are fine with water, especially during assessment and cutting).

Identifying Play-of-Colour: The Heart of Precious Opal

The most sought-after characteristic is, of course, the play-of-colour.

• Looking for Flashes: Turn the stone slowly under your light. Look for any flashes of color, even faint ones. These are your first clues.
• What is an Opal Colour Bar? Often, the precious opal (the part with colour) forms in a layer or band within the potch (common opal or host rock). This is known as the “colour bar.” Your goal is to identify its location, thickness, and quality.
• Direction of Colour: Note if the colour seems to face a particular direction. This will be crucial when planning your cut to maximize the display.

Finding the Colour Bar in Rough Opal

• Scan the Surface: Look for any windows or exposed areas where color is visible.
• Grind a Small Window (Carefully!): If the stone is completely encased in potch, experienced cutters might carefully grind a small test window on an inconspicuous edge to peek inside. This should be done with caution if you’re a beginner.
• Translucency Clues: Sometimes, holding a strong light behind a thinner piece of rough can reveal the shadow or density of a colour bar within.

Understanding Potch Lines and Their Significance

Potch is the common opal or host rock that doesn’t display play-of-colour. “Potch lines” can refer to the boundary between the precious opal and the potch, or veins of potch running through the colour bar.

• Clean Boundaries: A clean, distinct separation between the colour bar and the potch is often desirable.
• Potch Incursions: If potch lines run through the colour, they might necessitate cutting smaller stones or incorporating the pattern into the design. This isn’t always bad – sometimes it creates unique patterns!
• Thickness of the Colour Bar vs. Potch: A thick colour bar on a stable potch backing is ideal, but many beautiful opals are cut from thinner bars.

Spotting Inclusions, Cracks, and Flaws in Rough Opal

Not all that glitters is perfect. It’s vital to identify potential issues.

• Inclusions: These are any foreign materials or imperfections within the opal. They can be sand, matrix, gypsum, or other minerals.
  • Impact on Value: Significant or poorly placed inclusions can lower the value or make cutting difficult. However, some inclusions can create interesting patterns (e.g., “picture stones”).
• Cracks and Crazing: Look very carefully for fine lines or fractures.
  • Surface Cracks vs. Internal Cracks: Surface crazing might polish out, but deep cracks can mean the stone will fall apart during cutting or afterwards.
  • Stress Test (Use with Extreme Caution): Some experienced miners might gently squeeze a stone to check for instability, but this can easily break a fragile piece. It’s often better to assess visually.
• Sandstone or Matrix Content: Especially in boulder opal, the amount and type of host rock (ironstone, sandstone) are important. Is the opal well-adhered? Is the matrix stable?

Assessing Opal Clarity and Body Tone in the Rough

While play-of-colour is paramount, the clarity and body tone of the opal itself matter.

• Clarity: Is the part that will show colour clear, milky, or cloudy? Crystal opals will have high transparency, while white opals will be more opaque.
• Body Tone: This is the background colour of the opal, ranging from black (most valuable) through dark grey, light grey, to white. In rough, you’re trying to estimate what this will be once the stone is shaped.

Determining Potential Opal Orientation

How you orient the rough opal during cutting is critical to maximizing the play-of-colour.

• Follow the Bar: Generally, you want to cut the top of the stone parallel to the main colour bar.
• Face of the Colour: Identify which direction the colour “faces” best. This will become the top (table) of your finished gem.

Final Thoughts on Rough Opal Evaluation Techniques

Assessing rough opal potential is a skill honed with experience. Don’t be discouraged if your first few attempts are challenging.

• Practice: The more rough opal you examine, the better you’ll become.
• Ask Questions: If you’re buying from a reputable dealer like OBA, don’t hesitate to ask about their assessment of a particular piece or parcel.
• Embrace the Surprise: Part of the magic of opal cutting is the element of surprise. Sometimes the plainest looking rough yields a stunner!

By carefully inspecting for play-of-colour, understanding colour bars and potch, identifying inclusions and flaws, and considering orientation, you’ll be well on your way to making more informed decisions and hopefully uncovering some truly spectacular gems from your rough opal.