CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 101,402	$ 83,562
Trade receivables, net	29,242	22,979
Inventories, net	14,760	15,210
Prepaid expenses and other current assets	1,401	2,245
Total current assets	146,805	123,996
Property, plant and equipment	407,274	388,200
Less accumulated depreciation and depletion	(246,648)	(235,739)
Property, plant and equipment, net	160,626	152,461
Operating lease right-of-use assets	2,982	2,226
Other assets, net	495	415
Total assets	310,908	279,098
Current liabilities:
Accounts payable	5,712	4,592
Current portion of operating lease liabilities	903	1,187
Accrued expenses	5,243	5,809
Total current liabilities	11,858	11,588
Deferred tax liabilities, net	23,570	21,531
Operating lease liabilities, excluding current portion	2,116	1,030
Other liabilities	1,789	1,757
Total liabilities	39,333	35,906
Stockholders' equity:
Common stock	667	666
Additional paid-in capital	31,217	29,457
Retained earnings	294,903	268,186
Less treasury stock, at cost	(55,212)	(55,117)
Total stockholders' equity	271,575	243,192
Total liabilities and stockholders' equity	$ 310,908	$ 279,098