Hi Bamboo ~ I'd say it equates more to brilliant than extreme. (Extreme makes its emerald-greenness sound somehow excessive!) Other alternatives might be bright or pure ~ but, as always when talking about colours, it's not easy to pinpoint precisely what intensity (or brightness etc.) means without pointing at physical examples!
In technical terms, intense here means saturated. Colours are saturated when they're not wishy-washy.
But non-technical people would perhaps use strong. Since the word strength is used later in the sentence, referring to the physical strength of the stone, it would not be good to substitute strong for intense.