Howlite takes a dye very well, but not deeply. A broken specimen is usually white inside. Magnesite, on the other hand is usually blue throughout. And I suppose it may be that a lot of "magnesite" has been crushed to powder, colored, and reconstructed. (The astounding number of variations of "sea-sediment jasper" beads, that are just reconstructed fakes, of several sorts or varieties, has become invasive in the marketplace.)

Whether Howlite is routinely plasticized, I do not know.

However, this: In the 1970s Howlite, dyed blue for "turquoise" and red for "coral," was fairly common. Along came magnesite around twenty (20) years ago—and this was often called "Howlite"--even though Howlite imitations of turquoise were no longer circulating (as they had previously). This is still happening now. And I have to suppose that some authors cannot tell the differences, or don't care, since both fakes are mentioned in expositions. Howlite made a return to the marketplace only a few years ago—and was discussed at that time. (I will show a link in a reply.) So, once again, both imitations are available, and will probably continue to be confused with one-another.

Once a stone has been stabilized it is no longer porous, So the color is stable. The only likely damage would be from radiation--such as leaving it outside in the sun for some months. Other than that, the color is stable, and contaminants such as sweat, oil, perfume, or hairspray, probably will not affect the color.

Since most dyed turquoise has also been stabilized, the colors are more-or-less permanent. Likewise, magnesite, having been stabilized and/or reconstructed, is likewise stable as to color. I have not tested any Howlite to have a hands-on experience. So I cannot say whether the dye or colorant is permanent. Or if it can be destroyed by solvents or whatever else. My practical experience here is very limited. I just recognize Howlite when I see it (most of the time), and since I can tell it's a fake, I haven't considered much more. But when I have an opportunity to acquire some, I will test it.

Cheers, Jamey