how big of a crystal can you bring on board an air craft ?
 

how big can a crystal be and still be allowed on the airplane ?

Check the regulations of the company, don't ask us.
It depends on size & weight. If it's too heavy you'll have to pay and cannot be hand baggage.
Is it legal to export (if you leave a country) etc. Contact the airline.

51 gram piece moldavite on the plane i had no problems with smaller pieces so i should be able to bring it on board from the czech republic ?

I have no clue. You might have to inform with Czech customs.

thanks i will ask them.

That's one big Moldavite though and if you can get your hands on it go for it. Moldavite is in high demand and the market is swamped with fake ones. If you can get a genuine one in that size I can see why you would want to take it with you.

yeah i found a place that has a certificate from the government stating that it is real.

Definitely go for it. Would love to see a picture of it when you get it.

il be getting it the 15th of august when am in prague il post a picture here
the price is 1993 euros. or 51000 czech crowns.

Just a word of caution, 2 of the large gemstone dealers on ebay sell gemstones with 'legitimate certificates' and claim their gemstones are natural and are certified. Their products are made of glass, are not natural and of course are not certified. Oddly, ebay keeps them in business.