The different diamond

Ernie was always coming up with some quick scheme for money and Jack always knocked him down. Jack wondered if there was a time where Ernie would actually come up with some legitimate scheme. 

One bright summer’s day Jack set up shop. 

A local boy, Albert, came up. He placed his money on the table and announced, “I want to hire you.”

He started to explain: “See, Ernie is-”

5 seconds later, Jack was at Ernie’s den.

“Come see the region’s biggest diamond! Only 50p entry!” shouted Ernie. Ernie saw Jack and he scowled. 

“And here’s the local know-it-all!” crowed Ernie. “For him, half-price!” 

Jack got out 25p and walked to see the diamond. 

A sparkly blue gem twinkled, encased in dark scars. It was encased in mud and there was a small scar on its side. 

Albert, out of breath, appeared at Jack’s shoulder. 

“My gem-selling business and collection is going out of business. I suspect the gem isn’t real.” remarked Albert. 

Ernie, grinning, looked at Jack. 

“So whaddya think?”

“It’s a fake.” 

Why?