After reading about the malachite could be darken over times, I do some searches and find out something very intersting:
1. most of precious or semi-precious gemstones are treated and it is widely accepted as "natural" stones, except for diamond (must be stated in certificate if treated), alexandrite, garnet and spinel (no treatment at all). Some methods have been used for thousands years
2. all kinds of stones used in alhambra collection are quite fragile, with the hardness comes from 2.5 to 7 in Mohs scale (diamond is the hardest mineral with Mohs scale of 10 and chalk is the softest with only 1). I list them from the softest to the hardest stones, the numbers next to the stones is the Mohs
MOP 2.5-4.5
malachite, coral 3-4
lapis, turquoise 5-6
onyx, tiger-eye, carnelian 6.5-7
that's why some of these stones could be easily damaged when worn by scratching with harder stuff, eg. a copper penny has a hardness of 3; a knife blade, 5; window glass, 5.5; and a steel file, 6.5.
after doing my research I could "accept" that VCA has used "treatment" stones in their jewellry, but still could not accept the fact that the quality could be changed / worse over times (as mentioned here about malachite and lapis) as their pieces are mostly bought as "investment" not for a short time but long time of use. For such a hefty price tag, it should be remain the same in quality over times, other than that what is the heck of buying a luxury designer necklace if you could not pass it on to your daughters one day.
Cung, THANK YOU for doing this research and gathering all this information! It is so helpful! I am not really worried about malachite either, but it would be such a shame to accidentally damage a beautiful piece because I didn't understand its proper care. That said, VCA customer service is legendary and I am sure they would make it right!