CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Current assets:
Cash and cash equivalents	$ 108,581	$ 105,355
Trade receivables, net	37,123	26,715
Inventories, net	16,719	15,116
Prepaid expenses and other current assets	2,197	3,244
Total current assets	164,620	150,430
Property, plant and equipment	429,006	413,561
Less accumulated depreciation and depletion	(260,421)	(251,389)
Property, plant and equipment, net	168,585	162,172
Operating lease right-of-use assets	3,087	3,144
Other assets, net	441	450
Total assets	336,733	316,196
Current liabilities:
Accounts payable	7,100	5,433
Current portion of operating lease liabilities	794	899
Accrued expenses	4,522	4,856
Total current liabilities	12,416	11,188
Deferred tax liabilities, net	24,275	23,055
Operating lease liabilities, excluding current portion	2,420	2,311
Other liabilities	1,452	1,436
Total liabilities	40,563	37,990
Stockholders' equity:
Common stock	669	669
Additional paid-in capital	33,185	31,774
Retained earnings	318,250	301,611
Less treasury stock, at cost	(55,934)	(55,848)
Total stockholders' equity	296,170	278,206
Total liabilities and stockholders' equity	$ 336,733	$ 316,196