Those are really tough to seal. If I were trying to fill it I would soak the piece in acetone to be certain there is no oil left in the partially healed line. Let it dry under a shop light for a hour or two.
Mix and fill the line with Hxtal and let it sit for a week to cure. Hxtal is water clear and UV stable so it will not yellow as it ages. Since it takes days to set up and cure it will often (not always) soak down and fill these thin lines. I use it on similar ones in Lapis Lazuli and it often soaks down all the way through a slab. Try to keep it just on the fault line so air in the void can still migrate out of the stone.
I mix as little as four drops of Hxtal at a time. I keep it in a refrigerator and a small bottle lasts about a year.
Okay, so what is HXTAL and why is it so great?
HXTAL Epoxy Adhesive is an ultra pure resin that remains water white (clear and totally transparent) over both time and with extreme exposure to direct light. This is an exotic and very high-tech adhesive that is far more sophisticated than hardware store epoxy adhesives or even Opticon.
HXTAL is supplied in two liquid parts, both of low viscosity. Hxtal epoxy should be weighed out accurately, (one part by weight of Part B plus three parts by weight of Part A). This assures the maximum utilization of all the adhesive purchased.
The creator of HXTAL confirms that the mix ratio is not an extremely critical factor in the quality of the final achieved glue properties but an average of 3:1 is recommended for maximum control of overall consistency.
After the two parts have been weighed into a mixing jar, mix it thoroughly with a glass stirring rod. There is no rush during the mixing stage and there will also be plenty of time for the glue to sit while the bubbles rise and pop. Freshly mixed HXTAL is very thin and this is the time to remove the bubbles from the mixture. The best way to do this is to use some kind of a vacuum to suck the air out of the glue.
Freshly mixed HXTAL has a very thin viscosity. If it is too thin, let it stand (covered) and it will thicken over a period of several hours. The bond strength of thick or thin HXTAL is the same but various gluing applications may require varied viscosities of the HXTAL. Thin HXTAL will penetrate cracks for some repair applications, making them virtually disappear from view. The best results are obtained when the glass is warmed to about 120°F (a hair dryer or some other
heat source is reasonable if the object is not heated too much or too quickly). Then apply a drop of the freshly mixed HXTAL onto the crack. If the crack absorbs sufficient glue the crack will virtually disappear and the remaining HXTAL should be wiped off the surface only with a clean clothe or paper towel. Don’t forget that the glue in the crack will still take a long time to cure so just let it sit for a week before continuing any other work on that piece.
HXTAL sets slowly - at 75°F, it requires about one week to achieve most of the final bond strength, (see Physical Properties Sheet). However, ordinarily HXTAL is set sufficiently after 24 hours to hold the two parts together as long as no stress is applied to the glue joint. This is the best time to clean up if necessary. A single edge razor blade or an X-acto knife is a good tool to scrape off the excess resin. We don’t recommend the use of any solvents to clean up at this stage
because the solvents can migrate into the glue joint and can weaken the bond with the glass. The damage may not be evident until much later and will appear to be small bubbles at the edge of the joint. After 24 hours it will be extremely difficult to remove any excess glue from the object and grinding and polishing will be the next best way to remove it.
Many glass artists use HXTAL epoxy adhesive to glue pieces of various glass together to form art. http://www.hisglassworks.com/cart/cart.php?m=product_list&c=57