No planet is basically benefic or malefic. It all depends on the situation in which a planet is placed in a horoscope in different rasis (signs) and bhavas (houses) with respect to a particular ascendant ( lagna) at a particular time of birth. subjected  to various cross influences with respect to the placement of other planets. For example, the benefic or malefic tendency of a planet may differ as per its placement in exaltation/debilitation  rasi , own rasi/ star, friendly / enemy rasi/star, different bhavas as per its own lordship and as per its associations and aspects by other friendly / enemy planets etc. Each of these placements, aspects, associations are not of equal importance and they are to be considered giving different weightages for each type of influence. The following are some of the various types of benefic situations and influences that the planets are subjected to in a horoscope:

Benefic Analysis: if natural benefic, if lordship benefic, if in own sign, if in exaltation, if debilitated retrograde, if in functional friend's sign, if in lordship benefic's sign, if in natural benefic's sign, if in functional friend's star, if in own star, if in lordship benefic's star, if in natural benefic's star, if funcrtional friend of ascendant lord, if in association with functional friend, if in association with lordship benefic, if in association with natural benefic, if aspected by functional friend, if aspected by lordship benefic, if aspected by natural benefic, as natural malefic in 3rd, 6th, 10th or 11th bhavas, as natural benefic not in 12th bhava, as lordship malefic in 6th, 8th or 12th bhavas, as lordship benefic not in 6th, 8th or 12th bhavas,  as Vargottama i.e. same house in rasi and navamsa charts, in exaltation in navamsa chart, in own sign in navamsa chart, in functional friend's sign in navamsa chart.

Similarly   the Malefic analysis is to be done for various conditions. The details are given in my book "How to read a Horoscope- A Scientific Model of Predictions based on benefic & malefic analysis of planets and bhavas as per Hindu astrology" published by M/s Motilal Banarsidass Publishers Private Limited, New Delhi with ISBN : 81-208-1458-4(Cloth) and 81-208-1459-2(Paper)

Once the benefic and malefic percentage analysis is done, the corresponding gems for the benefic planets can be suggested to improve its performance, avoiding the gems for the malefic planets which may still adversely affect the malefic influences.

The gems to be used for the various planets, the minimum number of carats (above 30 years age), in gold or silver, in which finger , on which day to be first worn  are given below:

Sun-Ruby- 3 carats-in gold-right hand ring finger-Sunday
Moon-Pearl-3 carats-in silver-right hand ring finger-Monday
Mars-Red Coral-3 carats-in gold-right hand ring or index finger-Tuesday
Mercury-Green Emerald-3 carats-in gold-right hand little finger-Wednesday
Jupiter-Yellow Sapphire/Topaz-3 carats-in gold-right hand index finger-Thursday
Venus-Diamond-3 carats (or affordable since very costly)-in silver-right hand middle finger-Friday
Saturn-Blue Sappire-3 carats-in silver-right hand middle finger-Saturday
Rahu (Dragon's Head)-Gomed-3 carats-in silver-right hand middle finger-Saturday
Ketu (Dragon's Tail)-Cat's Eye-3 carats-in gold-right hand ring or index finger-Tuesday

( For more information contact  Pemmaraju V.R. Rayudu,67 Vinayanagar, Saidabad,Hyderabad-500059,Andhra Pradesh, India;  Phone: 040-4530210 ; email: rayadu@satyam.net.in or  pvrrayudu@hotmail.com or pvrrayudu@yahoo.com; website:  http://www.loudoun-net.com/kamesh/rayudu.html)