CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 91,347	$ 83,562
Trade receivables, net	27,547	22,979
Inventories, net	14,623	15,210
Prepaid expenses and other current assets	1,800	2,245
Total current assets	135,317	123,996
Property, plant and equipment	402,012	388,200
Less accumulated depreciation and depletion	(242,476)	(235,739)
Property, plant and equipment, net	159,536	152,461
Operating lease right-of-use assets	1,713	2,226
Other assets, net	546	415
Total assets	297,112	279,098
Current liabilities:
Accounts payable	5,309	4,592
Current portion of operating lease liabilities	914	1,187
Accrued expenses	4,382	5,809
Total current liabilities	10,605	11,588
Deferred tax liabilities, net	23,300	21,531
Operating lease liabilities, excluding current portion	813	1,030
Other liabilities	1,781	1,757
Total liabilities	36,499	35,906
Stockholders' equity:
Common stock	667	666
Additional paid-in capital	30,657	29,457
Retained earnings	284,501	268,186
Less treasury stock, at cost	(55,212)	(55,117)
Total stockholders' equity	260,613	243,192
Total liabilities and stockholders' equity	$ 297,112	$ 279,098