provides information on . . .

Make matrix type opals black
and prepare porous opals for polishing

TREATMENT for making Matrix Opals BLACK
sometimes refered to as "COOKING"

Ingredients and Equipment:
  • 1 packet of GLUCOSE or 1 lb. SUGAR
  • 1 quart container of SULFURIC acid 90% -98% concentration
  • 1 cooking pot
  • 1 medium size glass cooking bowl (pyrex)
The Process:
  1. Cut material into desired shape and thickness

  2. Dry the opal material either in oven, sun or with a hair dryer - (DO NOT OVER HEAT).

  3. Put sugar (glucose) in heated water (enough water to cover the matrix).

  4. Place the dried matrix stones into the solution.

  5. Cook in this solution, at a low temperature for 10 hours (DO NOT BOIL).

  6. Take out of solution and let dry for 24 hours.

  7. Pour concentrated Sulfuric acid into the glass cooking dish and heat at a low temperature (DO NOT BOIL AND PLACE IN A WELL VENTILATED AREA).

  8. Place carefully the matrix stones into the heated acid and cook for 3 hours at a low temperature.

  9. Take stones out and let cool, then wash in water.

  10. Place stones back into the sugar solution - Re-heat for 2 to 3 hours allowing any acid in the stones to be neutralized.

  11. Remove stones, wash in water to remove excess sugar.


Magnificent colors in porous type opals such as matrix opals are only present when you wet the stone. These opals will not take a normal polish without treatment. To remedy the situation, you may have to apply a polish lacquer to assist in the polishing. Even though treatment is frowned upon in gemstones, the results obtained are well worth it. Matrix opals should be made black before applying this lacquer treatment.

Ingredients and Equipment:
  • A clear 2-pack lacquer finish.
  • An appropriate thinner to clean equipment
  • A medium size art chisel type painting brush
  • Small glass container
  • Wet and dry sandpaper - 400 and 600 grit
  • Normal dopping equipment
The Process:
  1. Make sure the cut stone is dry and clean.

  2. Dop the cut stone as this will make it easier to handle, unless the stone is a larger specimen.

  3. Mix the 2-pack lacquer polish, following instructions as to the correct ratio. Measure out only what you need. Also important is that the room is well ventilated and is not too hot or too cold as this will effect the quality of the finish. When you have estimated the quantity you need, pour the correct potions of the 2-pack lacquer into a small glass bowl or jar and mix well.

  4. You can apply the polish lacquer in either of 3 ways:

    (a) Painting it on with a paint brush,

    (b) Spray painting it on, or

    (c) Dipping the stone into the mixture.

    [(a) and (c) are the preferred methods].

    Note: When applying the lacquer make sure that you remove the little air bubbles that appear by brushing them out. Let dry naturally for 24 hours or more.

  5. You will notice the surface is still a little pitted. This Is because the matrix has absorbed some of the lacquer. So after 24 hours repeat step 4.

  6. After waiting at least 48 hours to harden, you are ready to sand the surface of the stone using 400 grit sandpaper. This is done to sand out any bumps and pits.

  7. Repeat steps 4 and 5.

  8. Lightly sand the stone with 600 or 10OO grit.

  9. It is now time to polish the stone the same way you would a normaly polish an opal stone using cerium oxide or equivalent. If the result is still unsatisfactory repeat steps 4, 5, and 6.