@Anirban,
Diamond of 1, 2 or 3 carat would be very costly if you buy them in one piece (known as solitaire); 4 or 5 carat diamond would cost millions of ruppes as they are very very rare. Even, 1 carat diamond would cost lakhs or rupees. To give you an idea a 0.10 carat (also called 10 cent) real diamond can cost around Rs. 4,500 – way cheaper than pointer or solitaire diamonds. I am a gemologist and I deal in small diamonds (less than 15 cent). So, if you require a small good quality diamond set in gold or silver ring as suggested by astrologer then I can make it for you. Let me know if you’re interested.
Regarding the subsitute “white zircon” mentioned by astrologer; it is actually a synthetic gemstone made by man, popularly knonwn as zircon in the market but its real name is Synthetic Cubic Zirconia (also known as American Diamond, CZ, White Zircon in market). A 0.10 carat American Diamond would cost just Rs. 10 or less.
An advice – go with a real gemstone even if that is small rather than a man-made synthetic stone. You can get a diamond ring made with small diamonds (5/6/7 cent) with multiple piecies of around 16 so that would make it around 1 carat and it will still work for astrology and would be way much cheaper than a single 1 carat diamond.