From your date of birth, find the numbers, days, dates and colours that traditionally favour you.
None of it is arbitrary. Your numbers point at a ruling graha, and the graha brings its own weekday, its colours, its metal and its stone — Sun with Sunday and gold, Venus with Friday and white, Saturn with Saturday and blue-black. What this calculator does is trace that chain from your birth date and set it out plainly, so you can see the reasoning rather than a list of assertions.
A stone is the one item on this page that costs real money, so it gets the careful sentence. Classically a strengthening gem is worn only when its planet is benefic for your ascendant — and a date-based calculator cannot know your ascendant. If a site sells you a stone without having computed your lagna, it is selling a stone. The mantra, the weekday and the donation carry the same classical weight and cost nothing.
Not on the basis of this page. Cast the free chart first and check whether the planet is actually benefic for your ascendant — for some lagnas the same stone is specifically not advised.
Yes, and it is the cheapest of these to act on. It is a classical association rather than a rule, so use it where a choice is genuinely free and ignore it where it is not.