Pabloc, a few suggestions to start. Put a light source behind your rock and see if light passes through it. If it does, then it is translucent. It could be crystalline in nature and it sppears to be. The exciting part for you is it could be emerald. There were sunken galleys which had cargoes of emerald. If it is emerald, when you back light it, it will be green and it will be very hard. You will not be able to scratch the crystal with a metal file or a pocketknife. There are other possibilities. A quick way to find out would be a jeweler or a rock shop which may have a gem tester, and they could tell you.