The first thing I do is look to see if it is a handmade or machine made. That takes just a second. Then I look at the glass colors. Then I look at pattern.
While that may turn out to be the case - it isn't always. I had a contemporary marble maker rip me off for $500 and Paypal acknowledged the transaction but said I was on my own. I was out $500 and Paypal was completely useless.
Meanwhile - Mike Edmondson disappeared with my money.