Birthstone means the gemstone that you can wear all the time. It is your personal gemstone and can be worn without any hitch. What is that attracts and make you connected to your own birthstone? When you see it - there is some sparkle in eyes, when you search it among other gemstones. It is well connected to your own energies.
Your birthstones are well connected to you because they are well aligned to your energies. They are represented by your birth month/date or your astrological sign. They enhance your energy level and make you align to the environment by inducing the perfect wavelength to you. This makes you well connected to the higher cosmic level of the world or you can say macrocosm [as then you are microcosm]. This makes you feel the way as you are. As these birthstones are connected with your own astrological sign so they work on healing the mind, body and soul. On the connection with mind it brings clarity and helps to define your life purpose. When it relates to body- heal and provide energy to boost you up. Where it is related to soul- it helps your soul to go deeper to know yourself.
When you are well connected to it then you feel yourself important. You are more precious - not to others but to your own self. When you feel yourself important then a new confidence is felt within that makes you successful.
If you cannot afford costly gemstones then you can get their substitutes that are cheaper than the primary gemstones and also effective. See the list below and their effectiveness too that is assessed by my own personal experience.
Ruby can be substituted by Red Spinel
Pearl can be substituted by Moonstone that is very effective.
Red Coral is already a very cheap stone but if you wish to use the less stronger than coral then you can use Carnelian [raat ratua] instead of Red Coral
Emerald can be substituted by Peridot
Yellow Sapphire can be substituted by Yellow Citrine or Sunehla which is found far more effective in many cases.
Diamond is substituted by Zircon.
Blue Sapphire is substituted by Amethyst - known as katella in hindi or Blue sapphire is substituted by Iolite or Neeli.
Garnet already very cheap gemstone and is used frequently in jewellery better known as Gomedh in Hindi
Cat's eye is very strong and cheap gemstone. It is known as Lahsuniya in Hindi.
Remember that all gemstones are not suited to all. Consult an expert for best results. No two combinations should be worn without an expert's advice.